DevTools: revamped frame overview logic.

1. Show only one row of frames in Timeline's frame overview.
2. Only use impl-side frame signals as frame delitmiters for timeline overview.
3. Ignore impl-side frames that were not drawn or that happened while waiting for
the main thread frame, unless they also requested a main thread frame.
4. Account events that happened on the main thread to the impl-side frame that the
main thread frame was committed to.
5. In frame overview, if the CPU time of events accounted to a given frame exceeds
the wall time taken by the frame, normalize the colored bars length by frame
duration (these will thus show relative fractions of time spent per each category
of events);
6. Account background events (e.g. rasterization) to the impl-side frames similar to
how we did it for main thread.

Review URL: https://codereview.chromium.org/166653002

git-svn-id: svn://svn.chromium.org/blink/trunk@167227 bbb929c8-8fbe-4397-9dbb-9b2b20218538