Web Animations CSS: Support animation of background-image and -webkit-mask-image

background-image is not specified to be animatable however we are still
animating it to support legacy Blink/WebKit behaviour.

This implementation differs from the existing animation engine in the
following ways:
 - Interpolating between different counts of multiple background/mask
images behaves like a repeatable list interpolation.
 - Animating multiple mask images is supported.
 - The presence of keywords in interpolation defaults back to a 50% step
timing function.
The new test is aimed at the new implementation and will fail for the
current implementation, the Web Animation implementation passes the test.

BUG=257591

Review URL: https://codereview.chromium.org/27415003

git-svn-id: svn://svn.chromium.org/blink/trunk@160041 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Refine toRenderSVGResourceContainer()

As other toFooElements do, this patch checks security vulnerability in every use of toRenderSVGResourceContainer.
Plus, small binary size gain on content_shell(linux 64bit release build) by 0.17K
as it removes same function from RenderObject.

Review URL: https://codereview.chromium.org/27648002

git-svn-id: svn://svn.chromium.org/blink/trunk@160042 bbb929c8-8fbe-4397-9dbb-9b2b20218538