Web Animations: Address some review feedback to clean up CSS Animations

Addresses some additional feedback on: https://codereview.chromium.org/21012002

* Add assertions to clarify web-animations only code.
* Fix buggy conditional logic when calling setAnimatableStyle on RenderObject.

BUG=258896

Review URL: https://chromiumcodereview.appspot.com/22364002

git-svn-id: svn://svn.chromium.org/blink/trunk@155582 bbb929c8-8fbe-4397-9dbb-9b2b20218538