Bug 1281745, bug 1453206: Add a comment pointing out why shadow root checks are needed. r=me

As requested by Blake in bug 1281745.

DONTBUILD, because comment-only.
MozReview-Commit-ID: 1LX4ZlAedOh

dom/base/nsIContentInlines.h

@@ -176,6 +176,8 @@ nsIContent::IsActiveChildrenElement() const
     return false;
   }
 
+  // We reuse the binding parent machinery for Shadow DOM too, so prevent that
+  // from getting us confused in this case.
   return !bindingParent->GetShadowRoot();
 }
 
@@ -197,6 +199,8 @@ nsIContent::IsInAnonymousSubtree() const
     return false;
   }
 
+  // We reuse the binding parent machinery for Shadow DOM too, so prevent that
+  // from getting us confused in this case.
   return !bindingParent->GetShadowRoot();
 }