Revert of IntersectionObserverCallbackImpl should use Member<ExecutionContext>

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

Index: third_party/WebKit/Source/core/dom/IntersectionObserver.cpp
diff --git a/third_party/WebKit/Source/core/dom/IntersectionObserver.cpp b/third_party/WebKit/Source/core/dom/IntersectionObserver.cpp
index 5dc179ae350bbd66009dd9a5bb19d467b3400d3b..eade5171943781f148713d6be880359e56c06682 100644
--- a/third_party/WebKit/Source/core/dom/IntersectionObserver.cpp
+++ b/third_party/WebKit/Source/core/dom/IntersectionObserver.cpp
@@ -54,7 +54,7 @@
   }
 
  private:
-  Member<ExecutionContext> m_context;
+  WeakMember<ExecutionContext> m_context;
   std::unique_ptr<IntersectionObserver::EventCallback> m_callback;
 };
 
@@ -222,6 +222,7 @@
 Document& IntersectionObserver::trackingDocument() const {
   Document* document = nullptr;
   if (rootIsImplicit()) {
+    DCHECK(m_callback->getExecutionContext());
     document = toDocument(m_callback->getExecutionContext());
   } else {
     DCHECK(root());
@@ -309,8 +310,10 @@
 void IntersectionObserver::computeIntersectionObservations() {
   if (!rootIsValid())
     return;
-  LocalDOMWindow* callbackDOMWindow =
-      toDocument(m_callback->getExecutionContext())->domWindow();
+  Document* callbackDocument = toDocument(m_callback->getExecutionContext());
+  if (!callbackDocument)
+    return;
+  LocalDOMWindow* callbackDOMWindow = callbackDocument->domWindow();
   if (!callbackDOMWindow)
     return;
   DOMHighResTimeStamp timestamp =