media::BindToLoop() is born, with example uses to slim down WebMediaPlayerProxy.

media/base/bind_to_loop.h.pump

Index: media/base/bind_to_loop.h.pump
diff --git a/media/base/bind_to_loop.h.pump b/media/base/bind_to_loop.h.pump
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+$$ This is a pump file for generating file templates.  Pump is a python
+$$ script that is part of the Google Test suite of utilities.  Description
+$$ can be found here:
+$$
+$$ http://code.google.com/p/googletest/wiki/PumpManual
+$$
+
+$$ See comment for MAX_ARITY in base/bind.h.pump.
+$var MAX_ARITY = 7
+
+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef MEDIA_BASE_BIND_TO_LOOP_H_
+#define MEDIA_BASE_BIND_TO_LOOP_H_
+
+#include "base/bind.h"
+#include "base/callback_internal.h" // Avoid re-inventing CallbackForward.
+#include "base/location.h"
+#include "base/message_loop_proxy.h"
+
+// This is a helper utility for base::Bind()ing callbacks on to particular
+// MessageLoops.  A typical use is when |a| (of class |A|) wants to hand a
+// callback such as base::Bind(&A::AMethod, a) to |b|, but needs to ensure that
+// when |b| executes the callback, it does so on a particular MessageLoop.
+//
+// Typical usage: request to be called back on the current thread:
+// other->StartAsyncProcessAndCallMeBack(
+//    media::BindToLoop(MessageLoopProxy::current(),
+//                      base::Bind(&MyClass::MyMethod, this)));
+//
+// Note that like base::Bind(), BindToLoop() can't bind non-constant references,
+// and that *unlike* base::Bind(), BindToLoop() makes copies of its arguments,
+// and thus can't be used with arrays.
+
+namespace media {
+
+template <typename T> struct TrampolineHelper;
+
+$range ARITY 0..MAX_ARITY
+$for ARITY [[
+$range ARG 1..ARITY
+
+template <$for ARG , [[typename A$(ARG)]]>
+struct TrampolineHelper<void($for ARG , [[A$(ARG)]])> {
+  static void Run(
+      const scoped_refptr<base::MessageLoopProxy>& loop,
+      const base::Callback<void($for ARG , [[A$(ARG)]])>& cb
+$if ARITY != 0 [[, ]]
+$for ARG , [[A$(ARG) a$(ARG)]]
+) {
+    loop->PostTask(FROM_HERE, base::Bind(cb
+$if ARITY != 0 [[, ]]
+$for ARG , [[base::internal::CallbackForward(a$(ARG))]]));
+  }
+};
+
+
+]]  $$ for ARITY
+
+template<typename T>
+static base::Callback<T> BindToLoop(
+    const scoped_refptr<base::MessageLoopProxy>& loop,
+    const base::Callback<T>& cb) {
+  return base::Bind(&TrampolineHelper<T>::Run, loop, cb);
+}
+
+}  // namespace media
+
+#endif  // MEDIA_BASE_BIND_TO_LOOP_H_