Web Animations CSS: Support animation of background-position and background-image

Existing layout tests for background-image depend on background-size
animation being implemented before they can be enabled in TestExpectations.

The current implementation does not animate mis-matched background-position
value counts correctly and is expected to fail the newly added test.
The test passes under the Web Animations implementation.

Relevant spec:
http://dev.w3.org/csswg/css-backgrounds/#the-background-position
http://dev.w3.org/csswg/css-backgrounds/#the-background-image

BUG=257591

Review URL: https://codereview.chromium.org/26247003

git-svn-id: svn://svn.chromium.org/blink/trunk@159396 bbb929c8-8fbe-4397-9dbb-9b2b20218538