Use the stateless GC::fillRect variant when possible.

This reduces deferred paint save pressure and allows us to remove a
couple of GC state savers.

R=schenney@chromium.org,senorblanco@chromium.org
TBR=esprehn@chromium.org
BUG=424655

Review URL: https://codereview.chromium.org/671813002

git-svn-id: svn://svn.chromium.org/blink/trunk@184318 bbb929c8-8fbe-4397-9dbb-9b2b20218538