Avoid redundant checks for Compositing reasons

RenderObject can be any one of the Video, Canvas, Plugin or Iframe.

BUG=

Review URL: https://chromiumcodereview.appspot.com/22170002

git-svn-id: svn://svn.chromium.org/blink/trunk@155653 bbb929c8-8fbe-4397-9dbb-9b2b20218538