Web Animations CSS: Fix edge cases around fill-mode

* If the fill-mode is not set default to 'none' (default in Web Animations is forwards)
* Require a style recalc after removing an effect (to handle no forwards fill)
* Apply animation effect on the first frame. An erroneous condition meant that we were
  never doing this.

BUG=258896

Review URL: https://codereview.chromium.org/26367002

git-svn-id: svn://svn.chromium.org/blink/trunk@159181 bbb929c8-8fbe-4397-9dbb-9b2b20218538