DevTools: Support multiple target in TimelineModelImpl

BUG=

Review URL: https://codereview.chromium.org/397823003

git-svn-id: svn://svn.chromium.org/blink/trunk@178567 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Draw marker events on Timeline flame chart

Marker events are now displayed on the flame chart's timeline bar.

BUG=361045

Review URL: https://codereview.chromium.org/402113002

git-svn-id: svn://svn.chromium.org/blink/trunk@178568 bbb929c8-8fbe-4397-9dbb-9b2b20218538