Remove remaining traces of WebCompositorImpl from the blink source.

http://webk.it/97255 removed the WebCompositorImpl. This patch is just to remove
remaining trace of the forward declaration for it.

Review URL: https://codereview.chromium.org/111383002

git-svn-id: svn://svn.chromium.org/blink/trunk@163518 bbb929c8-8fbe-4397-9dbb-9b2b20218538