Revert of Web Animations: Timeline should not advance during task execution (https://codereview.chromium.org/275753002/)

Reason for revert:
Still failing Win7 debug:
http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Win7%20%28dbg%29/builds/5195

Original issue's description:
> Web Animations: Timeline should not advance during task execution
> 
> Previously the animation clock was only frozen during animation frame
> callbacks. This meant that the timeline was able to advance during other
> tasks.
> 
> This patch allows the clock to advance once per task, either to the
> compositor supplied frame start time or to an approximation of the next
> expected frame time.
> 
> http://dev.w3.org/fxtf/web-animations/#script-execution-and-live-updates-to-the-model
> 
> This is a reland of r173583[1], the previous patch hit added assertions
> due to double rounding issues, the new approach avoids these. Also ensures
> that other unit tests see a non-zero currentTime (seen in some webkit_unit_test
> runs on Android).
> 
> [1] https://src.chromium.org/viewvc/blink?view=rev&revision=173583
> BUG=367903
> TBR=abarth
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=173729

TBR=noel@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=367903

Review URL: https://codereview.chromium.org/275863002

git-svn-id: svn://svn.chromium.org/blink/trunk@173744 bbb929c8-8fbe-4397-9dbb-9b2b20218538