Web Animations: Rename KeyframeAnimationEffect to KeyframeEffectModel

The spec recently renamed KeyframeAnimationEffect to KeyframeEffect [1].
We add a 'Model' suffix because this class will not be exposed directly
to JavaScript. A wrapper class will be used to handle setFrames() and
getFrames() for invalid properties and property values.

[1]  Item 5 of the meeting minutes from 5th December 2013.
http://lists.w3.org/Archives/Public/public-fx/2013OctDec/0154.html

R=shans,rjwright

Review URL: https://codereview.chromium.org/107723003

git-svn-id: svn://svn.chromium.org/blink/trunk@164064 bbb929c8-8fbe-4397-9dbb-9b2b20218538