aw: Fix lost invalidate while playing video
BrowserViewRenderer::block_invalidates_ needs to be unset immediately
after draw. Posting the unset to UI thread could imply that it comes
after the fallback tick, which can cause an invalidate to be dropped.
This is especially bad for VideoLayer since the fallback tick does not
advance video frames, which means the video could be stuck forever.
Fix by unsetting block_invalidates_ and cancelling fallback tick
immediately, but only post back to get the invalidate.
Internal bug b/
14643271
BUG=
Review URL: https://codereview.chromium.org/
271163002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@270101 0039d316-1c4b-4281-b951-d872f2087c98