Support Microsoft::WRL::ComPtr as a receiver of base::Bind{Once,Repeating}

After this CL, we can use ComPtr as the |this| pointer of base::BindOnce
and base::BindRepeating.

ComPtr doesn't have `operator*` nor `operator->*` that are needed to run
a method pointer. So we need a special handling for it to use it as a
receiver on base::Bind.

We currently use scoped_refptr to store COM instance on base::Bind, which
happens to work, but that looks fragile. This CL is a preparation to
convert these scoped_refptr to ComPtr.

Change-Id: I1fe31e23ac627050860fe2c81ead6807a17bb939
Reviewed-on: https://chromium-review.googlesource.com/1145153
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577470}

base/bind_internal.h

@@ -46,6 +46,15 @@
 //  BindState<> -- Stores the curried parameters, and is the main entry point
 //                 into the Bind() system.
 
+#if defined(OS_WIN)
+namespace Microsoft {
+namespace WRL {
+template <typename>
+class ComPtr;
+}  // namespace WRL
+}  // namespace Microsoft
+#endif
+
 namespace base {
 
 template <typename T>
@@ -910,6 +919,13 @@ struct BindUnwrapTraits<internal::PassedWrapper<T>> {
   static T Unwrap(const internal::PassedWrapper<T>& o) { return o.Take(); }
 };
 
+#if defined(OS_WIN)
+template <typename T>
+struct BindUnwrapTraits<Microsoft::WRL::ComPtr<T>> {
+  static T* Unwrap(const Microsoft::WRL::ComPtr<T>& ptr) { return ptr.Get(); }
+};
+#endif
+
 // CallbackCancellationTraits allows customization of Callback's cancellation
 // semantics. By default, callbacks are not cancellable. A specialization should
 // set is_cancellable = true and implement an IsCancelled() that returns if the