Bug 1346623 - Allow anonymous content created with nsIDocument::InsertAnonymousContent can change from non-native to native AC

* Prevent canvas custom content from becoming NAC when reframing the root element
* Add an API to get computed style values through an AnonymousContent object

Tag #1375

dom/base/AnonymousContent.h

@@ -68,6 +68,11 @@ public:
                                 const Sequence<OwningNonNull<DOMRect>>& aRects,
                                 ErrorResult& aError);
 
+  void GetComputedStylePropertyValue(const nsAString& aElementId,
+                                     const nsAString& aPropertyName,
+                                     DOMString& aResult,
+                                     ErrorResult& aRv);
+
 private:
   ~AnonymousContent();
   nsCOMPtr<Element> mContentNode;