Timeline: fill frames according to CPU time spent per event category

Also add test for frame model. Note there are still discrepancies wrt
how time per category is computed for timeline records -- these will
be the subject of a separate patch.

BUG=361045

Review URL: https://codereview.chromium.org/320523004

git-svn-id: svn://svn.chromium.org/blink/trunk@175781 bbb929c8-8fbe-4397-9dbb-9b2b20218538