Correct variable name typo while accessing TimelineViewport object from TimelineTrackView object.
Correct variable name typo while accessing TimelineViewport object from TimelineTrackView object.

TimelineViewport object is stored as viewport_ in TimelineTrackView class.
In timeline_track_view.html it is referred as this.viewport_
in some places and in some places as this.viewport.

Changed all instances to this.viewport_.

BUG=None
R=nduca@chromium.org

Review URL: https://codereview.appspot.com/120380043
benschmaus/catapult,danbeam/catapult,danbeam/catapult,scottmcmaster/catapult,catapult-project/catapult,benschmaus/catapult,modulexcite/catapult,SummerLW/Perf-Insight-Report,catapult-project/catapult-csm,catapult-project/catapult,catapult-project/catapult-csm,zeptonaut/catapult,dstockwell/catapult,vmpstr/trace-viewer,catapult-project/catapult,dstockwell/catapult,SummerLW/Perf-Insight-Report,dstockwell/catapult,catapult-project/catapult-csm,benschmaus/catapult,benschmaus/catapult,catapult-project/catapult,SummerLW/Perf-Insight-Report,sahiljain/catapult,catapult-project/catapult-csm,scottmcmaster/catapult,catapult-project/catapult-csm,modulexcite/catapult,danbeam/catapult,0x90sled/catapult,modulexcite/catapult,sahiljain/catapult,danbeam/catapult,dstockwell/catapult,catapult-project/catapult,SummerLW/Perf-Insight-Report,vmpstr/trace-viewer,catapult-project/catapult-csm,sahiljain/catapult,catapult-project/catapult,catapult-project/catapult,0x90sled/catapult,zeptonaut/catapult,0x90sled/catapult,vmpstr/trace-viewer,zeptonaut/catapult,SummerLW/Perf-Insight-Report,sahiljain/catapult,catapult-project/catapult-csm,sahiljain/catapult,benschmaus/catapult,benschmaus/catapult,benschmaus/catapult,sahiljain/catapult,SummerLW/Perf-Insight-Report,scottmcmaster/catapult