Fix coverage rect scale.
Fix coverage rect scale.

This patch ensures to take ideal contents scale into account
when displaying the coverage tiles. This ensures that on high
dpi devices, the coverage rects appear correct.

R=dsinclair@chromium.org, nduca@chromium.org, dj2, nduca

Review URL: https://codereview.appspot.com/239410043

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