Some follow up to clean up the cl: https://chromium-review.googlesource.com/c/615023

1. Adding comments:
It's pretty non-obvious how each damage bool can get set though. It'd be nice to have comments explaining what each of the bools represent.
  bool layer_property_changed_not_from_property_trees_ : 1;
  bool layer_property_changed_from_property_trees_ : 1;

2. Further investigating potential bugs in which we should not reuse the cache.
There won't necessarily be another surface for the child animation. For example, a transform animation won't, in general, trigger surface creation. It might be that UI is always forcing surfaces and that's why we haven't seen bugs, but that's fragile. That said, you're right that this isn't new with this CL (though it could be that some of the over-damage that this CL is removing has been hiding that bug). 

3. Have a good mechanism to further separate the damage sources.
It'd be nice to explicitly capture damage from animations, by changing TransformTree::OnTransformAnimated and EffectTree::OnOpacityAnimated to set animation-related damage bools rather than the generic transform_changed and effect_changed. Then instead of having "FromPropertyTrees" and "NotFromPropertyTrees" versions of damage you could have "FromAnimation" and "NotFromAnimation".