Rename ThreadedAnimation to CompositorAnimations and switch to singleton pattern

Not to be submitted as is, dummy implementation and test should be removed.

Review URL: https://codereview.chromium.org/36573002

git-svn-id: svn://svn.chromium.org/blink/trunk@160419 bbb929c8-8fbe-4397-9dbb-9b2b20218538