In order to convert CSSPseudoElementType into its underlying type easier,
we define CSSPseudoElementTypeBase. However, keep using uint8_t directly for
forward declarations.
We will eventually use this in place of the various state flags stored on
AnimationCollection (e.g. mStyleRuleRefreshTime, mStyleChanging,
mHasPendingAnimationRestyle) as well as to do a more targetted update in
FlushAnimations and AddStyleUpdatesTo.
