Timeline: plumb RequestMainThreadFrame and DrawFrame to front-end

- add plumbing dor RequestMainThreadReame & DrawFrame from trace events
    to front-end;
- flush all pending background events before emitting a top-level main
    thread event.

R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/160323002

git-svn-id: svn://svn.chromium.org/blink/trunk@167163 bbb929c8-8fbe-4397-9dbb-9b2b20218538