Timeline: speed up TimelineFlameChart.setSelection

When user clicks on an item the view gets an event with entryIndex and
notifies TimelinePanel about that in term of records. As a result the
panel calls setSelection and it takes O(N) time to converts record back to entryIndex.

BUG=

Review URL: https://codereview.chromium.org/286203009

git-svn-id: svn://svn.chromium.org/blink/trunk@174178 bbb929c8-8fbe-4397-9dbb-9b2b20218538