There are still some other references to "source" in AnimationPlayer such as
HasInPlayerSource and UpdateSourceContent. These are renamed in a subsequent
patch (that doesn't require DOM peer review).
We define KeyframeEffectReadonly in KeyframeEffect.cpp since Web Animations also
defines KeyframeEffect and when we come to implement that I expect we'll define
it in the same class, maybe even using the same object.
This patch also adds a few missing includes in places where
KeyframeEffectReadonly is used so that we're not just cargo-culting it in.
