First step to remove wallClockTime from animation interface.

We are currently removing trying to remove all references to base::Time (wall
clock time) from Chrome/Blink compositor/graphics stack.

This requires multiple two-sided patches. Following the process found at
https://groups.google.com/a/chromium.org/d/msg/chromium-dev/ukaRczAK6t8/0ludyZ1Nu4cJ

The steps are;
 * (This Patch) -- Adding monotonic only API to Blink's animation delegate interface
   https://codereview.chromium.org/185633002

 * Making the delegate adapter use monotonic only API
   https://codereview.chromium.org/185643002

 * Remove the wallClockTime API from Blink/WebKit.
   https://codereview.chromium.org/185393005

BUG=299945

Review URL: https://codereview.chromium.org/185633002

git-svn-id: svn://svn.chromium.org/blink/trunk@168560 bbb929c8-8fbe-4397-9dbb-9b2b20218538