Display youtube placeholder in mutate phase (#13248)
Display youtube placeholder in mutate phase (#13248)

Detected with [black magic](https://github.com/ampproject/amphtml/pull/13223).
yieldbot/amphtml,aghassemi/amphtml,lannka/amphtml,clawr/amphtml,zhouyx/amphtml,techhtml/amphtml,sharethrough/amphtml,yieldmo/amphtml,clawr/amphtml,sriramkrish85/amphtml,BenDz/amphtml,zhangsu/amphtml,chaveznvg/amphtml,donttrustthisbot/amphtml,chaveznvg/amphtml,donttrustthisbot/amphtml,jpettitt/amphtml,honeybadgerdontcare/amphtml,media-net/amphtml,engtat/amphtml,smartadserver/amphtml,BenDz/amphtml,clawr/amphtml,sharethrough/amphtml,laterpay/amphtml,BenDz/amphtml,sebastianbenz/amphtml,jpettitt/amphtml,Gregable/amphtml,jridgewell/amphtml,jmadler/amphtml,danielrozenberg/amphtml,cory-work/amphtml,jridgewell/amphtml,media-net/amphtml,donttrustthisbot/amphtml,zhouyx/amphtml,pedjoni8/amphtml,ampproject/amphtml,yieldbot/amphtml,erwinmombay/amphtml,danielrozenberg/amphtml,src-code/amphtml,alanorozco/amphtml,camelburrito/amphtml,yieldbot/amphtml,honeybadgerdontcare/amphtml,zhangsu/amphtml,clawr/amphtml,jpettitt/amphtml,smartadserver/amphtml,alanorozco/amphtml,mhc-gh/amphtml,cvializ/amphtml,voyagegroup/amphtml,laterpay/amphtml,voyagegroup/amphtml,mhc-gh/amphtml,koshka/amphtml,luciancrasovan/amphtml,mhc-gh/amphtml,smartadserver/amphtml,erwinmombay/amphtml,tlong2/amphtml,koshka/amphtml,chaveznvg/amphtml,luciancrasovan/amphtml,chaveznvg/amphtml,sebastianbenz/amphtml,powdercloud/amphtml,sebastianbenz/amphtml,lannka/amphtml,yieldmo/amphtml,voyagegroup/amphtml,koshka/amphtml,donttrustthisbot/amphtml,pedjoni8/amphtml,donttrustthisbot/amphtml,yieldmo/amphtml,nexxtv/amphtml,sriramkrish85/amphtml,dotandads/amphtml,BenDz/amphtml,donttrustthisbot/amphtml,mhc-gh/amphtml,cvializ/amphtml,dotandads/amphtml,prateekbh/amphtml,techhtml/amphtml,tlong2/amphtml,sebastianbenz/amphtml,bpaduch/amphtml,tlong2/amphtml,cvializ/amphtml,camelburrito/amphtml,camelburrito/amphtml,aghassemi/amphtml,rsimha-amp/amphtml,chaveznvg/amphtml,voyagegroup/amphtml,src-code/amphtml,kamtschatka/amphtml,aghassemi/amphtml,lannka/amphtml,erwinmombay/amphtml,jridgewell/amphtml,rsimha-amp/amphtml,BenDz/amphtml,cvializ/amphtml,powdercloud/amphtml,pedjoni8/amphtml,clawr/amphtml,pedjoni8/amphtml,adup-tech/amphtml,kamtschatka/amphtml,mhc-gh/amphtml,cory-work/amphtml,camelburrito/amphtml,alanorozco/amphtml,danielrozenberg/amphtml,BenDz/amphtml,prateekbh/amphtml,sharethrough/amphtml,yieldmo/amphtml,powdercloud/amphtml,yandex-pcode/amphtml,bpaduch/amphtml,yandex-pcode/amphtml,powdercloud/amphtml,danielrozenberg/amphtml,prateekbh/amphtml,Gregable/amphtml,taboola/amphtml,jmadler/amphtml,media-net/amphtml,yandex-pcode/amphtml,nexxtv/amphtml,ampproject/amphtml,jpettitt/amphtml,camelburrito/amphtml,mhc-gh/amphtml,adup-tech/amphtml,erwinmombay/amphtml,danielrozenberg/amphtml,media-net/amphtml,kamtschatka/amphtml,luciancrasovan/amphtml,smartadserver/amphtml,widespace-os/amphtml,cvializ/amphtml,nexxtv/amphtml,engtat/amphtml,mhc-gh/amphtml,yandex-pcode/amphtml,rsimha-amp/amphtml,rsimha-amp/amphtml,danielrozenberg/amphtml,ampproject/amphtml,jpettitt/amphtml,pedjoni8/amphtml,jmadler/amphtml,sebastianbenz/amphtml,taboola/amphtml,ampproject/amphtml,engtat/amphtml,widespace-os/amphtml,lannka/amphtml,zhouyx/amphtml,yandex-pcode/amphtml,sharethrough/amphtml,jridgewell/amphtml,Gregable/amphtml,adup-tech/amphtml,rsimha-amp/amphtml,Gregable/amphtml,lannka/amphtml,ampproject/amphtml,bpaduch/amphtml,zhouyx/amphtml,sharethrough/amphtml,donttrustthisbot/amphtml,smartadserver/amphtml,aghassemi/amphtml,honeybadgerdontcare/amphtml,powdercloud/amphtml,src-code/amphtml,zhouyx/amphtml,media-net/amphtml,alanorozco/amphtml,nexxtv/amphtml,kamtschatka/amphtml,smartadserver/amphtml,danielrozenberg/amphtml,ampproject/amphtml,zhangsu/amphtml,zhouyx/amphtml,luciancrasovan/amphtml,honeybadgerdontcare/amphtml,prateekbh/amphtml,clawr/amphtml,dotandads/amphtml,prateekbh/amphtml,erwinmombay/amphtml,jpettitt/amphtml,laterpay/amphtml,jridgewell/amphtml,powdercloud/amphtml,widespace-os/amphtml,luciancrasovan/amphtml,lannka/amphtml,sebastianbenz/amphtml,alanorozco/amphtml,jmadler/amphtml,voyagegroup/amphtml,jpettitt/amphtml,mhc-gh/amphtml,voyagegroup/amphtml,bpaduch/amphtml,kamtschatka/amphtml,Gregable/amphtml,zhangsu/amphtml,yieldmo/amphtml,alanorozco/amphtml,honeybadgerdontcare/amphtml,cory-work/amphtml,jridgewell/amphtml,ampproject/amphtml,koshka/amphtml,yieldbot/amphtml,yieldmo/amphtml,voyagegroup/amphtml,nexxtv/amphtml,laterpay/amphtml,pedjoni8/amphtml,tlong2/amphtml,adup-tech/amphtml,kamtschatka/amphtml,ampproject/amphtml,src-code/amphtml,sriramkrish85/amphtml,prateekbh/amphtml,jridgewell/amphtml,sriramkrish85/amphtml,sharethrough/amphtml,taboola/amphtml,cory-work/amphtml,chaveznvg/amphtml,jmadler/amphtml,yandex-pcode/amphtml,taboola/amphtml,nexxtv/amphtml,rsimha-amp/amphtml,sriramkrish85/amphtml,adup-tech/amphtml,bpaduch/amphtml,zhouyx/amphtml,honeybadgerdontcare/amphtml,dotandads/amphtml,cvializ/amphtml,alanorozco/amphtml,Gregable/amphtml,taboola/amphtml,aghassemi/amphtml,techhtml/amphtml,Gregable/amphtml,koshka/amphtml,taboola/amphtml,dotandads/amphtml,alanorozco/amphtml,Gregable/amphtml,media-net/amphtml,aghass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