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

base/base.gypi

 # 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.
 
 {
   'target_defaults': {
     'variables': {
       'base_target': 0,
     },
     'target_conditions': [
@@ skipping 42 matching lines @@
           'basictypes.h',
           'bind.h',
           'bind_helpers.cc',
           'bind_helpers.h',
           'bind_internal.h',
           'bind_internal_win.h',
           'bits.h',
           'build_time.cc',
           'build_time.h',
           'callback.h',
+          'callback_helpers.h',
           'callback_internal.cc',
           'callback_internal.h',
           'cancelable_callback.h',
           'chromeos/chromeos_version.cc',
           'chromeos/chromeos_version.h',
           'command_line.cc',
           'command_line.h',
           'compiler_specific.h',
           'cpu.cc',
           'cpu.h',
@@ skipping 837 matching lines @@
             'third_party/xdg_mime/xdgmimemagic.c',
             'third_party/xdg_mime/xdgmimemagic.h',
             'third_party/xdg_mime/xdgmimeparent.c',
             'third_party/xdg_mime/xdgmimeparent.h',
           ],
         },
       ],
     }],
   ],
 }