Reviewed by Darin Adler.

        When properties are missing from animation keyframes, interpolate between those keyframes that specify them
        https://bugs.webkit.org/show_bug.cgi?id=40794

        When a property does not appear in all keyframes of a keyframe animation, we currently use the
        value of that property from the unanimated style. That forces the author to use additional
        keyframes for properties that need to "skip a keyframe", which is laborious.

        With this change, properties are interpolated between the keyframes in which they appear.
        This is equivalent to splitting each property out into its own set of keyframes.

        Tests: animations/missing-keyframe-properties-repeating.html
               animations/missing-keyframe-properties-timing-function.html
               animations/missing-keyframe-properties.html

        * css/CSSStyleSelector.h:
        * css/CSSStyleSelector.cpp:
        (WebCo...