folly/wangle/channel/ChannelPipeline.h: avoid -Wsign-compare errors (trivial)
folly/wangle/channel/ChannelPipeline.h: avoid -Wsign-compare errors (trivial)

Summary:
* folly/wangle/channel/ChannelPipeline.h: Declare for-loop indices
to be of type size_t(not int), to match size of upper bound.
Otherwise, gcc-4.9 fails with e.g.,
folly/wangle/channel/ChannelPipeline.h:126:23: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]

Test Plan:
Run this and note there are fewer errors than before:
fbconfig --platform-all=gcc-4.9-glibc-2.20 -r folly && fbmake dbgo

Reviewed By: davejwatson@fb.com

Subscribers: fugalh, folly-diffs@

FB internal diff: D1767783

Tasks: 5941250

Signature: t1:1767783:1420587925:d662b75e6a62ebd5bacdde28ad6e1da22ef777ac

mqeizi/folly,project-zerus/folly,guker/folly,gavioto/folly,upsoft/folly,loverszhaokai/folly,brunomorishita/folly,sakishum/folly,chjp2046/folly,fw1121/folly,mqeizi/folly,facebook/folly,hongliangzhao/folly,theiver9827/folly,zhiweicai/folly,ykarlsbrun/folly,SeanRBurton/folly,CJstar/folly,cole14/folly,bowlofstew/folly,charsyam/folly,colemancda/folly,mqeizi/folly,colemancda/folly,romange/folly,cppfool/folly,shaobz/folly,bowlofstew/folly,loversInJapan/folly,shaobz/folly,SammyK/folly,bobegir/folly,loverszhaokai/folly,lifei/folly,juniway/folly,loversInJapan/folly,CJstar/folly,zhiweicai/folly,reddit/folly,Orvid/folly,loverszhaokai/folly,doctaweeks/folly,arg0/folly,fw1121/folly,project-zerus/folly,KSreeHarsha/folly,fw1121/folly,floxard/folly,stonegithubs/folly,KSreeHarsha/folly,clearlylin/folly,bikong2/folly,CJstar/folly,wildinto/folly,loverszhaokai/folly,guker/folly,renyinew/folly,romange/folly,clearlinux/folly,cppfool/folly,chjp2046/folly,rklabs/folly,bowlofstew/folly,Eagle-X/folly,fw1121/folly,clearlinux/folly,alexst07/folly,facebook/folly,raphaelamorim/folly,colemancda/folly,sonnyhu/folly,bikong2/folly,theiver9827/folly,doctaweeks/folly,shaobz/folly,guker/folly,ykarlsbrun/folly,project-zerus/folly,yangjin-unique/folly,sonnyhu/folly,sonnyhu/folly,loverszhaokai/folly,clearlylin/folly,cppfool/folly,romange/folly,brunomorishita/folly,PoisonBOx/folly,lifei/folly,floxard/folly,SammyK/folly,Eagle-X/folly,arg0/folly,PPC64/folly,floxard/folly,juniway/folly,chjp2046/folly,gaoyingie/folly,arg0/folly,wildinto/folly,guker/folly,theiver9827/folly,sakishum/folly,gaoyingie/folly,KSreeHarsha/folly,cole14/folly,shaobz/folly,clearlinux/folly,PoisonBOx/folly,bikong2/folly,upsoft/folly,raphaelamorim/folly,PPC64/folly,alexst07/folly,rklabs/folly,kernelim/folly,Eagle-X/folly,clearlylin/folly,lifei/folly,yangjin-unique/folly,loversInJapan/folly,kernelim/folly,reddit/folly,tempbottle/folly,loversInJapan/folly,charsyam/folly,romange/folly,theiver9827/folly,upsoft/folly,sakishum/folly,SammyK/folly,Orvid/folly,nickhen/folly,raphaelamorim/folly,juniway/folly,tempbottle/folly,leolujuyi/folly,rklabs/folly,bsampath/folly,Hincoin/folly,renyinew/folly,leolujuyi/folly,zhiweicai/folly,kernelim/folly,theiver9827/folly,PPC64/folly,bowlofstew/folly,bobegir/folly,facebook/folly,reddit/folly,renyinew/folly,yangjin-unique/folly,bsampath/folly,facebook/folly,gaoyingie/folly,lifei/folly,bsampath/folly,facebook/folly,alexst07/folly,floxard/folly,xzmagic/folly,tomhughes/folly,stonegithubs/folly,yangjin-unique/folly,cole14/folly,Orvid/folly,nickhen/folly,kernelim/folly,clearlinux/folly,Eagle-X/folly,SeanRBurton/folly,renyinew/folly,bowlofstew/folly,tempbottle/folly,hongliangzhao/folly,guker/folly,nickhen/folly,Orvid/folly,chjp2046/folly,floxard/folly,gavioto/folly,alexst07/folly,ykarlsbrun/folly,sonnyhu/folly,sakishum/folly,Hincoin/folly,constantine001/folly,SeanRBurton/folly,PoisonBOx/folly,Hincoin/folly,gaoyingie/folly,cole14/folly,cppfool/folly,leolujuyi/folly,juniway/folly,tomhughes/folly,doctaweeks/folly,raphaelamorim/folly,kernelim/folly,tempbottle/folly,stonegithubs/folly,brunomorishita/folly,bikong2/folly,xzmagic/folly,leolujuyi/folly,cole14/folly,charsyam/folly,SammyK/folly,raphaelamorim/folly,tomhughes/folly,shaobz/folly,rklabs/folly,bobegir/folly,wildinto/folly,xzmagic/folly,project-zerus/folly,Hincoin/folly,gavioto/folly,sonnyhu/folly,SammyK/folly,leolujuyi/folly,reddit/folly,hongliangzhao/folly,xzmagic/folly,PPC64/folly,PoisonBOx/folly,chjp2046/folly,alexst07/folly,wildinto/folly,constantine001/folly,bsampath/folly,brunomorishita/folly,mqeizi/folly,zhiweicai/folly,PoisonBOx/folly,brunomorishita/folly,SeanRBurton/folly,bobegir/folly,colemancda/folly,Hincoin/folly,zhiweicai/folly,CJstar/folly,rklabs/folly,cppfool/folly,bikong2/folly,CJstar/folly,charsyam/folly,stonegithubs/folly,tomhughes/folly,PPC64/folly,renyinew/folly,ykarlsbrun/folly,mqeizi/folly,arg0/folly,SeanRBurton/folly,nickhen/folly,tomhughes/folly,bsampath/folly,bobegir/folly,KSreeHarsha/folly,clearlylin/folly,clearlylin/folly,tempbottle/folly,KSreeHarsha/folly,gavioto/folly,stonegithubs/folly,arg0/folly,yangjin-unique/folly,doctaweeks/folly,romange/folly,Orvid/folly,project-zerus/folly,ykarlsbrun/folly,constantine001/folly,doctaweeks/folly,xzmagic/folly,sakishum/folly,upsoft/folly,clearlinux/folly,wildinto/folly,gavioto/folly,nickhen/folly,gaoyingie/folly,fw1121/folly,Eagle-X/folly,lifei/folly,juniway/folly,colemancda/folly,upsoft/folly,constantine001/folly,hongliangzhao/folly,reddit/folly,hongliangzhao/folly,loversInJapan/folly,constantine001/folly