Re-use CSSParser logic to parse keyframe keys

This allows the CSSParser logic to be shared between ...
- Creating the keyframe from CSS
- Setting the keyText as a string from JavaScript

Also update StyleKeyframe to cache the string and vector representations of the keys.

This is purely an optimisation. No change in behavior.

R=apavlov

Review URL: https://chromiumcodereview.appspot.com/19037003

git-svn-id: svn://svn.chromium.org/blink/trunk@157426 bbb929c8-8fbe-4397-9dbb-9b2b20218538