DevTools: speedup timeline events overview draw

rendering and painting shared the same strip, so they never
get coalesced producing lots of fillRect calls.
Now they get coalesced if they share the same pixel.

Review URL: https://codereview.chromium.org/250493003

git-svn-id: svn://svn.chromium.org/blink/trunk@172587 bbb929c8-8fbe-4397-9dbb-9b2b20218538