Add default placeholder and fallback to all a4a examples. (#5553)
Add default placeholder and fallback to all a4a examples. (#5553)


tdrl/amphtml,sriramkrish85/amphtml,prateekbh/amphtml,kmh287/amphtml,powdercloud/amphtml,zhouyx/amphtml,sortable/amphtml,tlong2/amphtml,sriramkrish85/amphtml,sriramkrish85/amphtml,gumgum/amphtml,techhtml/amphtml,powdercloud/amphtml,lzanol/amphtml,camelburrito/amphtml,wjfang/amphtml,cvializ/amphtml,sortable/amphtml,kamtschatka/amphtml,zhucl/amphtml,powdercloud/amphtml,yandex-pcode/amphtml,jpettitt/amphtml,avimehta/amphtml,rajkumarsrk/amphtml,jpettitt/amphtml,keithwrightbos/amphtml,keithwrightbos/amphtml,voyagegroup/amphtml,Gregable/amphtml,yieldmo/amphtml,interspacejp/amphtml,Gregable/amphtml,yieldbot/amphtml,Mobusi-ads/amphtml,ampproject/amphtml,cory-work/amphtml,pietergreyling/amphtml,jpettitt/amphtml,gumgum/amphtml,sharethrough/amphtml,sortable/amphtml,media-net/amphtml,rifler/amphtml,honeybadgerdontcare/amphtml,pedjoni8/amphtml,comScore/amphtml,aghassemi/amphtml,nexxtv/amphtml,lannka/amphtml,sriramkrish85/amphtml,tdrl/amphtml,jlucero-contentad/amphtml,Adtoma/amphtml,lannka/amphtml,yandex-pcode/amphtml,voyagegroup/amphtml,ampproject/amphtml,danielrozenberg/amphtml,donttrustthisbot/amphtml,src-code/amphtml,prateekbh/amphtml,mhc-gh/amphtml,techhtml/amphtml,DAC-f1h-dev/amphtml,vmarteev/amphtml,Mobusi-ads/amphtml,koshka/amphtml,mhc-gh/amphtml,danielrozenberg/amphtml,DistroScale/amphtml,dotandads/amphtml,rajkumarsrk/amphtml,yieldmo/amphtml,laterpay/amphtml,honeybadgerdontcare/amphtml,ruturajv-media-net/amphtml,jdelhommeau/amphtml,arealit/amphtml,erwinmombay/amphtml,ampproject/amphtml,luciancrasovan/amphtml,pietergreyling/amphtml,lannka/amphtml,nexxtv/amphtml,mhc-gh/amphtml,smartadserver/amphtml,powdercloud/amphtml,avimehta/amphtml,kamtschatka/amphtml,jpettitt/amphtml,aghassemi/amphtml,smartadserver/amphtml,wjfang/amphtml,wjfang/amphtml,powdercloud/amphtml,widespace-os/amphtml,luciancrasovan/amphtml,erwinmombay/amphtml,camelburrito/amphtml,jlucero-contentad/amphtml,bpaduch/amphtml,laterpay/amphtml,cory-work/amphtml,zhangsu/amphtml,koshka/amphtml,spotxchange/amphtml,avimehta/amphtml,arealit/amphtml,tdrl/amphtml,donttrustthisbot/amphtml,BenDz/amphtml,burtcorp/amphtml,kmh287/amphtml,sortable/amphtml,tlong2/amphtml,vmarteev/amphtml,burtcorp/amphtml,zhouyx/amphtml,jlucero-contentad/amphtml,kixer-com/amphtml,kixer-com/amphtml,BenDz/amphtml,Mobusi-ads/amphtml,adiant/amphtml,adup-tech/amphtml,bpaduch/amphtml,sortable/amphtml,bpaduch/amphtml,BenDz/amphtml,kamtschatka/amphtml,burtcorp/amphtml,koshka/amphtml,cory-work/amphtml,rlugojr/amphtml,kmh287/amphtml,widespace-os/amphtml,taboola/amphtml,ruturajv-media-net/amphtml,yieldbot/amphtml,jmadler/amphtml,zhucl/amphtml,pietergreyling/amphtml,voyagegroup/amphtml,keithwrightbos/amphtml,jpettitt/amphtml,erwinmombay/amphtml,engtat/amphtml,adup-tech/amphtml,ecoron/amphtml,ruturajv-media-net/amphtml,Gregable/amphtml,dotandads/amphtml,zhucl/amphtml,kmh287/amphtml,nexxtv/amphtml,zhouyx/amphtml,media-net/amphtml,adiant/amphtml,ampproject/amphtml,triplelift/amphtml,honeybadgerdontcare/amphtml,jridgewell/amphtml,lzanol/amphtml,sharethrough/amphtml,alanorozco/amphtml,rajkumarsrk/amphtml,mhc-gh/amphtml,danielrozenberg/amphtml,zhucl/amphtml,alanorozco/amphtml,ruturajv-media-net/amphtml,nexxtv/amphtml,DAC-f1h-dev/amphtml,cory-work/amphtml,src-code/amphtml,widespace-os/amphtml,ecoron/amphtml,honeybadgerdontcare/amphtml,luciancrasovan/amphtml,bpaduch/amphtml,yieldbot/amphtml,DarXector/amphtml,danielrozenberg/amphtml,jlucero-contentad/amphtml,taboola/amphtml,arealit/amphtml,danielrozenberg/amphtml,shawnbuso/amphtml,media-net/amphtml,DarXector/amphtml,Adtoma/amphtml,keithwrightbos/amphtml,jridgewell/amphtml,nexxtv/amphtml,jpettitt/amphtml,yieldbot/amphtml,luciancrasovan/amphtml,mhc-gh/amphtml,pedjoni8/amphtml,spotxchange/amphtml,dotandads/amphtml,BenDz/amphtml,dotandads/amphtml,zhouyx/amphtml,zhouyx/amphtml,lzanol/amphtml,zhangsu/amphtml,comScore/amphtml,avimehta/amphtml,cvializ/amphtml,clawr/amphtml,widespace-os/amphtml,ecoron/amphtml,tlong2/amphtml,comScore/amphtml,erwinmombay/amphtml,mlb7687/amphtml,zhouyx/amphtml,danielrozenberg/amphtml,rsimha-amp/amphtml,rlugojr/amphtml,rlugojr/amphtml,voyagegroup/amphtml,sharethrough/amphtml,rifler/amphtml,honeybadgerdontcare/amphtml,cvializ/amphtml,sebastianbenz/amphtml,bpaduch/amphtml,voyagegroup/amphtml,adiant/amphtml,mlb7687/amphtml,engtat/amphtml,luciancrasovan/amphtml,taboola/amphtml,techhtml/amphtml,yieldmo/amphtml,zhangsu/amphtml,jdelhommeau/amphtml,aghassemi/amphtml,yandex-pcode/amphtml,src-code/amphtml,jridgewell/amphtml,donttrustthisbot/amphtml,jridgewell/amphtml,ADITION/amphtml,fullspeed-sp/amphtml,Adtoma/amphtml,Gregable/amphtml,clawr/amphtml,sklobovskaya/amphtml,yieldmo/amphtml,laterpay/amphtml,zhucl/amphtml,nexxtv/amphtml,pedjoni8/amphtml,rsimha-amp/amphtml,kmh287/amphtml,DarXector/amphtml,pedjoni8/amphtml,sebastianbenz/amphtml,danielrozenberg/amphtml,gumgum/amphtml,luciancrasovan/amphtml,jmadler/amphtml,sklobovskaya/amphtml,chaveznvg/amphtml,tdrl/amphtml,engtat/amphtml,DistroScale/amphtml,keithwrightbos/amphtml,rajkumarsrk/amphtml,widespace-os/amphtml,fullspeed-sp/amphtml,kamtschatka/amphtml,chaveznvg/amphtml,triplelift/amphtml,zhangsu/amphtml,smartadserver/amphtml,keithwrightbos/amphtml,rlugojr/amphtml,yandex-pcode/amphtml,cory-work/amphtml,chaveznvg/amphtml,donttrustthisbot/amphtml,BenDz/amphtml,ampproject/amphtml,cvializ/amphtml,sharethrough/amphtml,engtat/amphtml,ecoron/amphtml,media-net/amphtml,DistroScale/amphtml,jmadler/amphtml,interspacejp/amphtml,comScore/amphtml,laterpay/amphtml,jridgewell/amphtml,wjfang/amphtml,kixer-com/amphtml,tdrl/amphtml,prateekbh/amphtml,kixer-com/amphtml,zhouyx/amphtml,powdercloud/amphtml,cory-work/amphtml,techhtml/amphtml,chaveznvg/amphtml,fullspeed-sp/amphtml,widespace-os/amphtml,clawr/amphtml,lannka/amphtml,lzanol/amphtml,camelburrito/amphtml,comScore/amphtml,rlugojr/amphtml,sklobovskaya/amphtml,yieldmo/amphtml,jmadler/amphtml,wjfang/amphtml,DAC-f1h-dev/amphtml,cvializ/amphtml,adup-tech/amphtml,ADITION/amphtml,adup-tech/amphtml,mhc-gh/amphtml,alanorozco/amphtml,arealit/amphtml,sebastianbenz/amphtml,alanorozco/amphtml,src-code/amphtml,media-net/amphtml,ruturajv-media-net/amphtml,ampproject/amphtml,DistroScale/amphtml,kixer-com/amphtml,jdelhommeau/amphtml,keithwrightbos/amphtml,JuliusThms/amphtml,voyagegroup/amphtml,vmarteev/amphtml,kmh287/amphtml,triplelift/amphtml,bpaduch/amphtml,cory-work/amphtml,interspacejp/amphtml,gumgum/amphtml,interspacejp/amphtml,camelburrito/amphtml,adiant/amphtml,src-code/amphtml,taboola/amphtml,kixer-com/amphtml,triplelift/amphtml,ecoron/amphtml,laterpay/amphtml,sharethrough/amphtml,mlb7687/amphtml,jmadler/amphtml,mlb7687/amphtml,jdelhommeau/amphtml,arealit/amphtml,burtcorp/amphtml,techhtml/amphtml,adiant/amphtml,lzanol/amphtml,bpaduch/amphtml,prateekbh/amphtml,JuliusThms/amphtml,DAC-f1h-dev/amphtml,sklobovskaya/amphtml,pietergreyling/amphtml,alanorozco/amphtml,koshka/amphtml,yieldmo/amphtml,Gregable/amphtml,rifler/amphtml,lannka/amphtml,tlong2/amphtml,spotxchange/amphtml,fullspeed-sp/amphtml,BenDz/amphtml,jmadler/amphtml,powdercloud/amphtml,burtcorp/amphtml,rajkumarsrk/amphtml,pedjoni8/amphtml,mlb7687/amphtml,triplelift/amphtml,jdelhommeau/amphtml,tlong2/amphtml,interspacejp/amphtml,alanorozco/amphtml,kamtschatka/amphtml,luciancrasovan/amphtml,media-net/amphtml,pedjoni8/amphtml,gumgum/amphtml,gumgum/amphtml,aghassemi/amphtml,vmarteev/amphtml,taboola/amphtml,media-net/amphtml,DarXector/amphtml,engtat/amphtml,DistroScale/amphtml,arealit/amphtml,jridgewell/amphtml,vmarteev/amphtml,ampproject/amphtml,Gregable/amphtml,yandex-pcode/amphtml,jlucero-contentad/amphtml,lzanol/amphtml,shawnbuso/amphtml,sebastianbenz/amphtml,danielrozenberg/amphtml,DarXector/amphtml,engtat/amphtml,adup-tech/amphtml,tdrl/amphtml,Adtoma/amphtml,mhc-gh/amphtml,alanorozco/amphtml,lannka/amphtml,engtat/amphtml,clawr/amphtml,ampproject/amphtml,zhucl/amphtml,rsimha-amp/amphtml,jdelhommeau/amphtml,Mobusi-ads/amphtml,DistroScale/amphtml,prateekbh/amphtml,sebastianbenz/amphtml,sriramkrish85/amphtml,erwinmombay/amphtml,rajkumarsrk/amphtml,kamtschatka/amphtml,jlucero-contentad/amphtml,JuliusThms/amphtml,shawnbuso/amphtml,fullspeed-sp/amphtml,ruturajv-media-net/amphtml,comScore/amphtml,nexxtv/amphtml,rifler/amphtml,rajkumarsrk/amphtml,yieldbot/amphtml,jpettitt/amphtml,spotxchange/amphtml,honeybadgerdontcare/amphtml,alanorozco/amphtml,rifler/amphtml,yandex-pcode/amphtml,JuliusThms/amphtml,lannka/amphtml,donttrustthisbot/amphtml,smartadserver/amphtml,donttrustthisbot/amphtml,adiant/amphtml,burtcorp/amphtml,media-net/amphtml,avimehta/amphtml,shawnbuso/amphtml,honeybadgerdontcare/amphtml,clawr/amphtml,DAC-f1h-dev/amphtml,prateekbh/amphtml,mhc-gh/amphtml,sortable/amphtml,ADITION/amphtml,clawr/amphtml,spotxchange/amphtml,zhucl/amphtml,zhangsu/amphtml,dotandads/amphtml,mlb7687/amphtml,taboola/amphtml,triplelift/amphtml,rifler/amphtml,ADITION/amphtml,comScore/amphtml,aghassemi/amphtml,adiant/amphtml,shawnbuso/amphtml,fullspeed-sp/amphtml,rsimha-amp/amphtml,gumgum/amphtml,chaveznvg/amphtml,vmarteev/amphtml,avimehta/amphtml,pietergreyling/amphtml,kixer-com/amphtml,camelburrito/amphtml,shawnbuso/amphtml,yieldbot/amphtml,DarXector/amphtml,cvializ/amphtml,voyagegroup/amphtml,smartadserver/amphtml,Mobusi-ads/amphtml,interspacejp/amphtml,aghassemi/amphtml,tlong2/amphtml,techhtml/amphtml,JuliusThms/amphtml,JuliusThms/amphtml,koshka/amphtml,DAC-f1h-dev/amphtml,sklobovskaya/amphtml,pietergreyling/amphtml,clawr/amphtml,sriramkrish85/amphtml,camelburrito/amphtml,sebastianbenz/amphtml,camelburrito/amphtml,jridgewell/amphtml,sharethrough/amphtml,spotxchange/amphtml,arealit/amphtml,Gregable/amphtml,Adtoma/amphtml,wjfang/amphtml,Adtoma/amphtml,wjfang/amphtml,dotandads/amphtml,src-code/amphtml,vmarteev/amphtml,yieldmo/amphtml,rsimha-amp/amphtml,Gregable/amphtml,ecoron/amphtml,Mobusi-ads/amphtml,sklobovskaya/amphtml,rsimha-amp/amphtml,adup-tech/amphtml,smartadserver/amphtml,sklobovskaya/amphtml,chaveznvg/amphtml,ADITION/amphtml,jpettitt/amphtml,laterpay/amphtml,yandex-pcode/amphtml,zhangsu/amphtml,shawnbuso/amphtml,rlugojr/amphtml,prateekbh/amph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