Reviewed by Darin Adler

        https://bugs.webkit.org/show_bug.cgi?id=23219

        Add support for transtions and animations of background-position, -webkit-background-size,
        -webkit-mask-position and -webkit-mask-size.

        Tests: transitions/background-transitions.html
               transitions/mask-transitions.html

        * page/animation/AnimationBase.cpp:
        (WebCore::blendFunc):
        Add a blend func for LengthSize.

        (WebCore::ensurePropertyMap):
        Add wrappers for the new properties.

        (WebCore::addShorthandProperties):
        Add a shorthand property for CSSPropertyBackgroundPosition to map to
        CSSPropertyBackgroundPositionX and CSSPropertyBackgroundPositionY,
        add CSSPropertyWebkitMask for CSSPropertyWebkitMaskPosition, and add
        CSSPropertyWebkitMaskPosition for CSSPropertyWebkitMaskPositionX and
        CSSPropertyWebkitMaskPositionY.

        * rendering/style/RenderS...