Update CSSAnimationUpdate and CSSAnimations to handle multiple animations with a given name

This will be required to handle CSS animations with incomplete keyframes, which we will handle by splitting the animation up into multiple InertAnimation objects.

Uploading this preparatory work as a separate change to simplify the change for incomplete keyframes.

No functional change.

BUG=240653
R=dstockwell

Review URL: https://chromiumcodereview.appspot.com/25079006

git-svn-id: svn://svn.chromium.org/blink/trunk@158516 bbb929c8-8fbe-4397-9dbb-9b2b20218538