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..17dbfa3a762d2bfe3cfb44f80a1ae3c103a451dc 100644
--- a/base/bind_internal.h
+++ b/base/bind_internal.h
@@ -319,11 +319,11 @@ struct InvokeHelper<true, void> {
   static void MakeItSo(Runnable&& runnable,
                        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)...);
+        weak_ptr, std::forward<RunArgs>(args)...);

[danakj, 2016/06/10 23:36:18]

I had to look at a bunch of places to understand why weak_ptr didn't need to be
moved here. Run takes a Receiver&&, but BoundWeakPtr will actually be a const&
since that is what Unwrap returns, IIUC.

It might be more obvious/less hidden, if you make |weak_ptr| be a forwarding
reference like the other arguments to this method with a &&, and forward it
here?

[tzik, 2016/06/14 12:06:10]

Done.

   }
 };