Emit CommitFrame and BeginFrame for impl-side frames

This adds processing of CommitFrame and BeginFrame to TimelineTraceEventProcessor.

BUG=305863
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/59793003

git-svn-id: svn://svn.chromium.org/blink/trunk@163167 bbb929c8-8fbe-4397-9dbb-9b2b20218538