Changing animate to beginFrame and use struct rather than naked double to allow extension.

This change makes it significantly easier to extend the interface for the needs for the Blink scheduler, web animations engine and future similar changes. It also brings the concept in Blink closer to the values in Chrome.

BUG=346230

Review URL: https://codereview.chromium.org/321373003

git-svn-id: svn://svn.chromium.org/blink/trunk@178493 bbb929c8-8fbe-4397-9dbb-9b2b20218538