Add DocumentShutdown{Notifier,Observer}

third_party/WebKit/Source/core/dom/Document.cpp

Index: third_party/WebKit/Source/core/dom/Document.cpp
diff --git a/third_party/WebKit/Source/core/dom/Document.cpp b/third_party/WebKit/Source/core/dom/Document.cpp
index 61e9adeb3203d5c6948e4f18c22edcda46cf3448..617f37cf60a602ee7d2d22143edc836d7c849926 100644
--- a/third_party/WebKit/Source/core/dom/Document.cpp
+++ b/third_party/WebKit/Source/core/dom/Document.cpp
@@ -2639,6 +2639,7 @@ void Document::Shutdown() {
   GetPage()->GetEventHandlerRegistry().DocumentDetached(*this);
 
   // Signal destruction to mutation observers.
+  DocumentShutdownNotifier::NotifyContextDestroyed();
   SynchronousMutationNotifier::NotifyContextDestroyed();
 
   // If this Document is associated with a live DocumentLoader, the
@@ -6858,6 +6859,7 @@ DEFINE_TRACE(Document) {
   ContainerNode::Trace(visitor);
   ExecutionContext::Trace(visitor);
   SecurityContext::Trace(visitor);
+  DocumentShutdownNotifier::Trace(visitor);
   SynchronousMutationNotifier::Trace(visitor);
 }