Use callback_forward.h instead of callback.h where possible.

base/callback_forward.h

Index: base/callback_forward.h
diff --git a/base/callback_forward.h b/base/callback_forward.h
new file mode 100644
index 0000000000000000000000000000000000000000..0f62864b3181f56264d68b6d453d23d1005da257
--- /dev/null
+++ b/base/callback_forward.h
@@ -0,0 +1,18 @@
+// Copyright (c) 2011 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 BASE_CALLBACK_FORWARD_H_
+#define BASE_CALLBACK_FORWARD_H_
+#pragma once
+
+namespace base {
+
+template <typename Sig>
+class Callback;
+
+typedef Callback<void(void)> Closure;
+
+}  // namespace base
+
+#endif  // BASE_CALLBACK_FORWARD_H