Make Callback::Reset() return a copy to support use-cases where Run() ends up modifying |*this|.

base/callback_helpers.h

Index: base/callback_helpers.h
diff --git a/base/callback_helpers.h b/base/callback_helpers.h
new file mode 100644
index 0000000000000000000000000000000000000000..52cb71bbf2610d7e359a20b9250581511ea368c7
--- /dev/null
+++ b/base/callback_helpers.h
@@ -0,0 +1,30 @@
+// 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.
+
+// This defines helpful methods for dealing with Callbacks.  Because Callbacks
+// are implemented using templates, with a class per callback signature, adding
+// methods to Callback<> itself is unattractive (lots of extra code gets
+// generated).  Instead, consider adding methods here.
+//
+// ResetAndReturn(&cb) is like cb.Reset() but allows executing a callback (via a
+// copy) after the original callback is Reset().  This can be handy if Run()
+// reads/writes the variable holding the Callback.
+
+#ifndef BASE_CALLBACK_HELPERS_H_
+#define BASE_CALLBACK_HELPERS_H_
+
+#include "base/callback.h"
+
+namespace base {
+
+template <typename Sig>
+base::Callback<Sig> ResetAndReturn(base::Callback<Sig>* cb) {
+  base::Callback<Sig> ret(*cb);
+  cb->Reset();
+  return ret;
+}
+
+}  // namespace base
+
+#endif  // BASE_CALLBACK_HELPERS_H_