Fix fading out media controls

This patch fixes the media controls panel to fade out, instead of appear
instantly. This behaviour was broken in blink r175153 which changed how
we stored the parsed data for CSS transitions.

The fade in behaviour has actually been broken for a long time, since
CSS transitions will not start if the before style has display set to
none. I've not tried to fix this issue in this patch.

BUG=398279

Review URL: https://codereview.chromium.org/445893002

git-svn-id: svn://svn.chromium.org/blink/trunk@179622 bbb929c8-8fbe-4397-9dbb-9b2b20218538