Add a hasVisibleContent getter to slice_track
Add a hasVisibleContent getter to slice_track

BUG=

Review URL: https://codereview.appspot.com/11058043
benschmaus/catapult,vmpstr/trace-viewer,catapult-project/catapult-csm,vmpstr/trace-viewer,SummerLW/Perf-Insight-Report,SummerLW/Perf-Insight-Report,0x90sled/catapult,scottmcmaster/catapult,catapult-project/catapult-csm,catapult-project/catapult,modulexcite/catapult,modulexcite/catapult,zeptonaut/catapult,dstockwell/catapult,sahiljain/catapult,danbeam/catapult,catapult-project/catapult,benschmaus/catapult,dstockwell/catapult,benschmaus/catapult,vmpstr/trace-viewer,catapult-project/catapult-csm,catapult-project/catapult,dstockwell/catapult,danbeam/catapult,SummerLW/Perf-Insight-Report,sahiljain/catapult,0x90sled/catapult,catapult-project/catapult-csm,catapult-project/catapult,modulexcite/catapult,0x90sled/catapult,danbeam/catapult,sahiljain/catapult,benschmaus/catapult,benschmaus/catapult,SummerLW/Perf-Insight-Report,sahiljain/catapult,zeptonaut/catapult,catapult-project/catapult,danbeam/catapult,catapult-project/catapult-csm,catapult-project/catapult,sahiljain/catapult,SummerLW/Perf-Insight-Report,scottmcmaster/catapult,SummerLW/Perf-Insight-Report,catapult-project/catapult,dstockwell/catapult,scottmcmaster/catapult,catapult-project/catapult-csm,catapult-project/catapult-csm,benschmaus/catapult,sahiljain/catapult,benschmaus/catapult,zeptonaut/catapult