Web Animations CSS: Avoid recursive event dispatch

This was happening under the test runner where a call to
testRunner.notifyDone() was dumping pixels and recursively forcing an
animation update.

BUG=258896

Review URL: https://chromiumcodereview.appspot.com/23008003

git-svn-id: svn://svn.chromium.org/blink/trunk@155997 bbb929c8-8fbe-4397-9dbb-9b2b20218538