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

git-svn-id: 3a56fcae908c7e16d23cb53443ea4795ac387cf2@1435 0e6d7f2b-9903-5b78-7403-59d27f066143

bpsinc-native/src_third_party_trace-viewer,bpsinc-native/src_third_party_trace-viewer,bpsinc-native/src_third_party_trace-viewer,bpsinc-native/src_third_party_trace-viewer