Introduce base::IsWeakReceiver for base::Bind to support external weak pointers

base/bind_internal.h

Index: base/bind_internal.h
diff --git a/base/bind_internal.h b/base/bind_internal.h
index 4da13eed99d20a001e08c564f3fd7818d9549955..d998491b60e2bb208537e830eed710a8d7859172 100644
--- a/base/bind_internal.h
+++ b/base/bind_internal.h
@@ -317,13 +317,13 @@ template <>
 struct InvokeHelper<true, void> {
   template <typename Runnable, typename BoundWeakPtr, typename... RunArgs>
   static void MakeItSo(Runnable&& runnable,
-                       BoundWeakPtr weak_ptr,
+                       BoundWeakPtr&& weak_ptr,
                        RunArgs&&... args) {
-    if (!weak_ptr.get()) {
+    if (!weak_ptr) {
       return;
     }
     std::forward<Runnable>(runnable).Run(
-        weak_ptr.get(), std::forward<RunArgs>(args)...);
+        std::forward<BoundWeakPtr>(weak_ptr), std::forward<RunArgs>(args)...);
   }
 };