Don't show video fullscreen until DidEnterFullscreen() is called

For fullscreen video on android, We call showFullscreenOverlay() too early.
We should wait until DidEnterFullscreen() gets called, and then we should let the video element to request the surface view.
Otherwise, if enterFullscreenForElement fails, we will incorrectly showing the fullscreen video in the background.
This CL also removes the hideFullscreenOverlay() call since this is no longer needed.
When exiting fullscreen, video view will automatically be destroyed.

BUG=367346

Review URL: https://codereview.chromium.org/277823002

git-svn-id: svn://svn.chromium.org/blink/trunk@173917 bbb929c8-8fbe-4397-9dbb-9b2b20218538