Implement Custom Elements' attributeChangedCallback.

Source/core/dom/CustomElementCallbackDispatcher.h

Index: Source/core/dom/CustomElementCallbackDispatcher.h
diff --git a/Source/core/dom/CustomElementCallbackDispatcher.h b/Source/core/dom/CustomElementCallbackDispatcher.h
index c37b5ca94e37e8581f97ee98e6689a7284263814..915c96a8cee19b7449f33d86cef5c1d1b9eafc75 100644
--- a/Source/core/dom/CustomElementCallbackDispatcher.h
+++ b/Source/core/dom/CustomElementCallbackDispatcher.h
@@ -66,6 +66,7 @@ public:
         size_t m_savedElementQueueStart;
     };
 
+    void enqueueAttributeChangedCallback(PassRefPtr<CustomElementLifecycleCallbacks>, PassRefPtr<Element>, const AtomicString& name, const AtomicString& oldValue, const AtomicString& newValue);
     void enqueueCreatedCallback(PassRefPtr<CustomElementLifecycleCallbacks>, PassRefPtr<Element>);
 
     // Returns true if more work may have to be performed at the
@@ -102,6 +103,9 @@ private:
     static void processElementQueueAndPop();
     void processElementQueueAndPop(size_t start, size_t end);
 
+    CustomElementCallbackQueue* createCallbackQueue(PassRefPtr<CustomElementLifecycleCallbacks>, PassRefPtr<Element>);
+    CustomElementCallbackQueue* ensureCallbackQueue(PassRefPtr<CustomElementLifecycleCallbacks>, PassRefPtr<Element>);
+
     // The processing stack, flattened. Element queues lower in the
     // stack appear toward the head of the vector. The first element
     // is a null sentinel value.