TimelineFlameChart: fix for exception in highlightTimeRange which happens when entryIndex is incorrect.

It happens when newly recorded timeline has fewer elements than the old timeline.
So we need to reset FlameChart when we start the new recording session.

BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/216713002

git-svn-id: svn://svn.chromium.org/blink/trunk@170308 bbb929c8-8fbe-4397-9dbb-9b2b20218538