RenderWidget shouldn't track its clip rect

Previously, RenderWidget tracked its clip rect in order to send clipRectChanged
messages to plugins. In a previous CL, I removed the clipRectChanged
notification. In this CL, I'm removing the clip rect tracking entirely.
Removing this tracking resolves a compositing chicken/egg problem because we
needed to know compositing state to compute our clip rects.

R=esprehn@chromium.org
BUG=339898

Review URL: https://codereview.chromium.org/160983002

git-svn-id: svn://svn.chromium.org/blink/trunk@167049 bbb929c8-8fbe-4397-9dbb-9b2b20218538