Enables opacity, transform and filter animations to be run by the compositor.

CSSPendingAnimations is introduced to manage the synchronized start of
compositor and main-thread animations.

RenderLayerCompositor is updated to query the Web Animations model for cases
where it should composite a layer.

ActiveAnimations can be queried by property to test whether animations are
running, or running on the compositor.

Note that these changes are still behind the WebAnimationsCSS runtime flag.

There will be some subsequent changes to address missing functionality:
* pauseForTesting is not wired up for animations on the compositor
* CSS Transitions do not start from an up to date value when the target is updated
* layers are not always composited as they are in the current implementation

BUG=311423,298211

Review URL: https://codereview.chromium.org/23874019

git-svn-id: svn://svn.chromium.org/blink/trunk@162269 bbb929c8-8fbe-4397-9dbb-9b2b20218538