Fix accalerated 2D canvas not explicitly invalidating the canvas content layer on update.

The the call to RenderObject::invalidatePaintRectangle in
HTMLCanvasElement only propagates to the canvas' containing layer
and not the child canvas layer that actually has the canvas content
in the case of an accelerated canvas. This is not a problem as far as
graphics updates are concerned, but it fails to activate the
compositor's rate limiter. The regression was caused by r176815 and
continued to exist after r179637 (patches that refactor canvas
invalidations).

BUG=404955

Review URL: https://codereview.chromium.org/462403003

git-svn-id: svn://svn.chromium.org/blink/trunk@180796 bbb929c8-8fbe-4397-9dbb-9b2b20218538