Re-land "Drop background color optimization for composited layers"

The original CL r168245 was reverted by r168317 because of crashes.
It's because r168245 removes null-check of document().documentElement()->renderer()
in updating-compositing-descendant-geometry process because the CL assumed all
elements own renderer at that time. However, it's not true on Mac.

So this CL does not remove the null-check to land.

BUG=114658, 340691
R=alokp@chromium.org, enne@chromium.org

Review URL: https://codereview.chromium.org/183833020

git-svn-id: svn://svn.chromium.org/blink/trunk@168634 bbb929c8-8fbe-4397-9dbb-9b2b20218538