Revert of Web Animations: Remove AnimationPlayer discarding logic

Revert of Web Animations: Remove AnimationPlayer discarding logic (patchset #2 id:20001 of https://codereview.chromium.org/955303002/)

Reason for revert:
It looks like this patch broke `webkit_unit_tests` (`WebFrameSwapTest.SwapPreservesGlobalContext` in particular) on a number of platforms. https://build.chromium.org/p/chromium.webkit/builders/WebKit%20Linux/builds/44832 for example.

Original issue's description:
> Web Animations: Remove AnimationPlayer discarding logic
> 
> Since forwards filling animations are supposed to be returned via
> getAnimationPlayers, we can no longer discard them when we've dropped
> references to them. This patch removes this discarding logic.
> 
> BUG=439247
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=190977

TBR=dstockwell@chromium.org,samli@chromium.org,timloh@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=439247
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=190983

[Mike West, 2015-02-27 10:46:11]

Created Revert of Web Animations: Remove AnimationPlayer discarding logic

[commit-bot: I haz the power, 2015-02-27 10:46:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/967523002/1

[commit-bot: I haz the power, 2015-02-27 10:47:22]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=190983