Web Animations: Remove timeLag from Player API and model

API changes are guarded by experimental feature flag. No visible changes to CSS
animations and element.animate.

This is a prototype for behavior we plan to incorporate into the Web Animations
specification:
* startTime is effective start time or NaN when paused or starting
* timeLag is removed

Review URL: https://codereview.chromium.org/365163004

git-svn-id: svn://svn.chromium.org/blink/trunk@178619 bbb929c8-8fbe-4397-9dbb-9b2b20218538