Add a BeginMainFrame fallback to MainThreadEventQueue.

I believe we aren't always getting the begin main frame signal leading
to an increase in hung renderers when rAF mouse aligned input is
enabled. Test my theory by adding a timer that fires after a
significantly long time (5s) if we are waiting for a signal but
haven't received it.

BUG=703344

Review-Url: https://codereview.chromium.org/2846333002
Cr-Commit-Position: refs/heads/master@{#468464}
4 files changed