Fix rounding errors on ruler.
Fix rounding errors on ruler.

When formatting for display it's more appropriate to use Math.round
instead of Math.floor due to multiply/divide errors.

BUG=624
R=nduca@chromium.org

Review URL: https://codereview.appspot.com/223880043

catapult-project/catapult-csm,danbeam/catapult,modulexcite/catapult,sahiljain/catapult,catapult-project/catapult,catapult-project/catapult-csm,danbeam/catapult,zeptonaut/catapult,danbeam/catapult,scottmcmaster/catapult,catapult-project/catapult-csm,dstockwell/catapult,catapult-project/catapult-csm,catapult-project/catapult-csm,benschmaus/catapult,SummerLW/Perf-Insight-Report,benschmaus/catapult,catapult-project/catapult,vmpstr/trace-viewer,catapult-project/catapult,benschmaus/catapult,SummerLW/Perf-Insight-Report,SummerLW/Perf-Insight-Report,catapult-project/catapult,catapult-project/catapult,danbeam/catapult,zeptonaut/catapult,catapult-project/catapult,zeptonaut/catapult,modulexcite/catapult,scottmcmaster/catapult,dstockwell/catapult,catapult-project/catapult-csm,catapult-project/catapult-csm,0x90sled/catapult,0x90sled/catapult,sahiljain/catapult,benschmaus/catapult,catapult-project/catapult,benschmaus/catapult,dstockwell/catapult,sahiljain/catapult,scottmcmaster/catapult,sahiljain/catapult,sahiljain/catapult,dstockwell/catapult,SummerLW/Perf-Insight-Report,vmpstr/trace-viewer,SummerLW/Perf-Insight-Report,benschmaus/catapult,0x90sled/catapult,modulexcite/catapult,vmpstr/trace-viewer,benschmaus/catapult,SummerLW/Perf-Insight-Report,sahiljain/catapult