Make HTMLMediaElement use setNeedsCompositingUpdate instead of scheduleLayerUpdate

Since RenderVideo always has a RenderLayer we can directly ask for a compositing
update instead of going through scheduleLayerUpdate.

Review URL: https://codereview.chromium.org/261753014

git-svn-id: svn://svn.chromium.org/blink/trunk@173364 bbb929c8-8fbe-4397-9dbb-9b2b20218538