Bug 1334051 - Part 2: Invoke attributeChangedCallback only if attribute name is in the observed attribute list.

We call attributeChangedCallback in two cases:

1. When any of the attributes in the observed attribute list has changed, appended, removed, or replaced.

2. When upgrading an element, for each attribute in element's attribute list that is in the observed attribute list.

Note: w/ Fixup for not implementing an API Enhancement Bug 1363481.

Tag UXP Issue #1344

dom/base/nsContentUtils.cpp

@@ -9637,6 +9637,28 @@ nsContentUtils::SetupCustomElement(Element* aElement,
   return registry->SetupCustomElement(aElement, aTypeExtension);
 }
 
+/* static */ CustomElementDefinition*
+nsContentUtils::GetElementDefinitionIfObservingAttr(Element* aCustomElement,
+                                                    nsIAtom* aExtensionType,
+                                                    nsIAtom* aAttrName)
+{
+  nsString extType = nsDependentAtomString(aExtensionType);
+  NodeInfo *ni = aCustomElement->NodeInfo();
+
+  CustomElementDefinition* definition =
+    LookupCustomElementDefinition(aCustomElement->OwnerDoc(), ni->LocalName(),
+                                  ni->NamespaceID(),
+                                  extType.IsEmpty() ? nullptr : &extType);
+
+  // Custom element not defined yet or attribute is not in the observed
+  // attribute list.
+  if (!definition || !definition->IsInObservedAttributeList(aAttrName)) {
+    return nullptr;
+  }
+
+  return definition;
+}
+
 /* static */ void
 nsContentUtils::EnqueueLifecycleCallback(nsIDocument* aDoc,
                                          nsIDocument::ElementCallbackType aType,