[CSS Background Blending] Test background blending with a gif layer over a background color

Adding test case.

BUG=251727

TBR=krit
(manual commit for https://codereview.chromium.org/23205013/)

Review URL: https://codereview.chromium.org/23455056

git-svn-id: svn://svn.chromium.org/blink/trunk@157970 bbb929c8-8fbe-4397-9dbb-9b2b20218538