Use ConvertTarget() to convert Nullabe<ElementOrCSSPseudoElement> into
Maybe<OwningAnimationTarget>, so SetTarget() can use it as well. The return type
of ConvertTarget is not Maybe<NonOwningAnimationTarget> because we need to
compare the return type with KeyframeEffectReadOnly::mTarget, so using the same
type would be easier to implement.
MozReview-Commit-ID: GskbydOMoo0
1. Define OwningAnimationTarget.
2. Replace the KeyframeEffectReadOnly::mTarget/mPseudoType members with
a Maybe<OwningAnimationTarget> mTarget member.
MozReview-Commit-ID: 65qOoNyDRSy
