Adds a CancellableTaskFactory and exposes postLoadingTasks to Blink

Source/wtf/WeakPtr.h

Index: Source/wtf/WeakPtr.h
diff --git a/Source/wtf/WeakPtr.h b/Source/wtf/WeakPtr.h
index 900e14cc0c43f8d6a3e7907879916b520c426c6b..608e9532689321a63b7a5d75fd4f2e8b1d85fa6f 100644
--- a/Source/wtf/WeakPtr.h
+++ b/Source/wtf/WeakPtr.h
@@ -140,6 +140,11 @@ public:
         m_ref = WeakReference<T>::create(ptr);
     }
 
+    bool hasWeakPtrs() const
+    {
+        return m_ref->refCount() > 1;
+    }
+
 private:
     RefPtr<WeakReference<T>> m_ref;
 };