Web Animations: Plug in PauseAnimationsForTesting

Hook pause for testing up to testing/Internals.

BUG=271815

Review URL: https://chromiumcodereview.appspot.com/22377008

git-svn-id: svn://svn.chromium.org/blink/trunk@155985 bbb929c8-8fbe-4397-9dbb-9b2b20218538