Refactoring RenderLayerCompositor to avoid redundant checks for Compositing reasons.

A renderer can be of type either Video, Canvas, Plugin or Iframe.

Review URL: https://chromiumcodereview.appspot.com/18005002

git-svn-id: svn://svn.chromium.org/blink/trunk@153464 bbb929c8-8fbe-4397-9dbb-9b2b20218538