[CSS Blending] Background color isn't fully painted when having a blended child.
The problem seems to be the creation of a transparency layer while
painting the background. The solution is to prevent any form of isolation
caused by blending when painting the root element.

BUG=310693

Review URL: https://codereview.chromium.org/37483004

git-svn-id: svn://svn.chromium.org/blink/trunk@160444 bbb929c8-8fbe-4397-9dbb-9b2b20218538