CSS Animations: Animations should start immediately when the page is not visible

In r67865[1] the starting of CSS animations and transitions was deferred
until the next compositing update. This introduced a regression where
animations triggered on a page which is not visible would not start
until the page was made visible again.

This patch allows animations to start immediately when a page is not visible.

[1] https://src.chromium.org/viewvc/blink?view=rev&revision=167865

BUG=350892

Review URL: https://codereview.chromium.org/225093015

git-svn-id: svn://svn.chromium.org/blink/trunk@171021 bbb929c8-8fbe-4397-9dbb-9b2b20218538