updateGraphicsLayerGeometry should cache clip rects

This CL makes the compositing update in toggle_drawer 10% faster by
caching clip rects in updateGraphicsLayerGeometry. We already have
a mechanism for caching clip rects. We just weren't using it in this
situation.

It's unclear why the clip cache is correctly invalidated for this
line of code. I spent several hours trying to write a test case that
uses stale clip rects, but I wasn't able to construct one. It's likely
we're just overly agressive about invalidating clip rects.

In this CL, I've added an ASSSERT that the clip rects aren't dirty. If
we find a test case that triggers this ASSERT, it's easy to clean the
clip rects at that point. However, I didn't want to add the cleaning
call without a test case that demonstrates that it's necessary.

NOTRY=true

Review URL: https://codereview.chromium.org/182603002

git-svn-id: svn://svn.chromium.org/blink/trunk@168104 bbb929c8-8fbe-4397-9dbb-9b2b20218538