Timeline Trace viewer prototype

This adds a timeline mode for recording a trace and a very basic trace
viewer based on FlameChart.

BUG=

Review URL: https://codereview.chromium.org/212683005

git-svn-id: svn://svn.chromium.org/blink/trunk@170548 bbb929c8-8fbe-4397-9dbb-9b2b20218538