Refactoring animation code in accelerated path.

Introduce WebAnimationProvider to extract animation logic from
RenderLayerBacking and GraphicsLayer.

The goals of this CL are
1. Get together animation logic in one file to increase readability.
2. Resolve layer violation of GraphicsLayer that includes CSS.

BUG=N/A

Review URL: https://chromiumcodereview.appspot.com/23431021

git-svn-id: svn://svn.chromium.org/blink/trunk@158052 bbb929c8-8fbe-4397-9dbb-9b2b20218538