Add support to resolve unprefixed CSS animations properties.

This is the start of various patches to support unprefixed CSS Animations.
The approach here is the same used for the CSS Transitions :
- Both sets of properties are added to the StylePropertySet which keep them
in sync.
- Only the prefixed ones are resolved.

Added a parsing tests to cover the parsing behavior in detail.

Review URL: https://chromiumcodereview.appspot.com/22925002

git-svn-id: svn://svn.chromium.org/blink/trunk@156034 bbb929c8-8fbe-4397-9dbb-9b2b20218538