Fix for crashing win/aura browser test

Fix by making Canvas2DLayerBridge flush and exit early when an accelerated canvas is being composited by a compositor with a software renderer.

TEST=NetInternalsTest.netInternalsTimelineViewScrollbar
BUG=288870
R=senorblanco

Review URL: https://chromiumcodereview.appspot.com/23546010

git-svn-id: svn://svn.chromium.org/blink/trunk@157618 bbb929c8-8fbe-4397-9dbb-9b2b20218538