Revert of [Reland] Web Animations: Introduce String based KeyframeEffectModel (https://codereview.chromium.org/217143004/)

Reason for revert:
Speculative revert due to 103 animation tests down: http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Win7%20%28dbg%29/builds/4529

Original issue's description:
> [Reland] Web Animations: Introduce String based KeyframeEffectModel
> 
> Note: This is a rework of issue 212023004
> 
> This patch refactors KeyframeEffectModel to support different types of
> keyframe values. The common keyframe logic is retained in
> KeyframeEffectModelBase to be inherited by the keyframe type specialisations.
> 
> KeyframeEffectModel is then templated on Keyframe type, with a hierarchy of Keyframes
> including StringKeyframe and AnimatableValueKeyframe being introduced.
> 
> String based keyframes are being introduced to properly support the JS API
> for Web Animations element.animate().
> 
> Original issue: https://codereview.chromium.org/215883005
> 
> TBR=rafaelw,dstockwell
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=170720

TBR=dstockwell@chromium.org,rafaelw@chromium.org,shans@chromium.org
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/222983006

git-svn-id: svn://svn.chromium.org/blink/trunk@170732 bbb929c8-8fbe-4397-9dbb-9b2b20218538