Delete MaskImageChanged, VideoChanged, and FullScreenChanged

* MaskImageChanged: No one did anything based on this notification.
* VideoChanged: There's only want caller and it's simpler to just
  setNeedsCompositingUpdate, which does the same thing.
* FullScreenChanged: No one ever sent this notification.

Review URL: https://codereview.chromium.org/334463002

git-svn-id: svn://svn.chromium.org/blink/trunk@175972 bbb929c8-8fbe-4397-9dbb-9b2b20218538