Implement CSSOM of unprefixed @keyframes.

This commit implements the bits for the CSSOM to work unprefixed:

-We now define the RuleType KEYFRAME_RULE and KEYFRAMES_RULE and make the
old prefixed versions equal to them.
- WebKitCSSKeyframe(s)Rule.idl are now renamed to be unprefixed.

Due to the way the IDL bindings works there is a behavior change. When querying the
keyframes rule the type of the object will always be [object CSSKeyframeRule] no matter
if prefixed or not and no matter if the flag is on or off (the old value was
[object WebKitCSSKeyframeRule]).

Window.WebKitCSSKeyframe(s)Rule is now undefined, Window.CSSKeyframe(s)Rule is now
defined (even if the feature flag is off).

Review URL: https://chromiumcodereview.appspot.com/22867036

git-svn-id: svn://svn.chromium.org/blink/trunk@156600 bbb929c8-8fbe-4397-9dbb-9b2b20218538