Facilitate rename of WebAnimation to WebCompositorAnimation

Typedef WebAnimation to WebCompositorAnimation.
Typedef WebAnimationCurve to WebCompositorAnimationCurve.

This is patch 1 of 3. Subsequent patches are http://crrev.com/413983002 and http://crrev.com/412123002
 
BUG=396358

Review URL: https://codereview.chromium.org/414643002

git-svn-id: svn://svn.chromium.org/blink/trunk@179379 bbb929c8-8fbe-4397-9dbb-9b2b20218538