Do not throw an error if a picture cannot be rasterized
Do not throw an error if a picture cannot be rasterized

This patch fixes a bug where pictureAsImageData could be null when
the benchmarking extension is not enabled, causing an error to be
printed to the console.

TBR=dsinclair
BUG=344

Review URL: https://codereview.appspot.com/12001048
dstockwell/catapult,benschmaus/catapult,sahiljain/catapult,SummerLW/Perf-Insight-Report,benschmaus/catapult,0x90sled/catapult,zeptonaut/catapult,modulexcite/catapult,catapult-project/catapult,sahiljain/catapult,danbeam/catapult,dstockwell/catapult,vmpstr/trace-viewer,scottmcmaster/catapult,catapult-project/catapult-csm,benschmaus/catapult,catapult-project/catapult,SummerLW/Perf-Insight-Report,scottmcmaster/catapult,benschmaus/catapult,modulexcite/catapult,SummerLW/Perf-Insight-Report,SummerLW/Perf-Insight-Report,benschmaus/catapult,scottmcmaster/catapult,catapult-project/catapult-csm,catapult-project/catapult,vmpstr/trace-viewer,dstockwell/catapult,benschmaus/catapult,catapult-project/catapult,SummerLW/Perf-Insight-Report,modulexcite/catapult,sahiljain/catapult,benschmaus/catapult,SummerLW/Perf-Insight-Report,catapult-project/catapult-csm,sahiljain/catapult,vmpstr/trace-viewer,dstockwell/catapult,catapult-project/catapult,catapult-project/catapult,danbeam/catapult,0x90sled/catapult,catapult-project/catapult-csm,sahiljain/catapult,catapult-project/catapult-csm,zeptonaut/catapult,danbeam/catapult,0x90sled/catapult,zeptonaut/catapult,danbeam/catapult,catapult-project/catapult,sahiljain/catapult,catapult-project/catapult-csm,catapult-project/catapult-csm