Timeline: show impl-side frames on the Timeline overview

Expose impl-side frames as additional row of vertical bars
on top of Timeline frame mode overview.

BUG=305863

Review URL: https://codereview.chromium.org/61923003

git-svn-id: svn://svn.chromium.org/blink/trunk@163000 bbb929c8-8fbe-4397-9dbb-9b2b20218538