Make the browser->metro_viewer IPC channel name a constant instead of hardcoding it in a few places…

win8/win8.gyp

 # 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.
 {
   'variables': {
     'chromium_code': 1,
   },
   'includes': [
     '../build/win_precompile.gypi',
   ],
@@ skipping 42 matching lines @@
     {
       'target_name': 'metro_viewer',
       'type': 'static_library',
       'dependencies': [
         '../base/base.gyp:base',
         '../ipc/ipc.gyp:ipc',
         '../ui/aura/aura.gyp:aura',
         '../ui/metro_viewer/metro_viewer.gyp:metro_viewer_messages',
       ],
       'sources': [
+        'viewer/metro_viewer_constants.cc',
+        'viewer/metro_viewer_constants.h',
         'viewer/metro_viewer_process_host.cc',
         'viewer/metro_viewer_process_host.h',
       ],
     },
     {
       'target_name': 'test_support_win8',
       'type': 'static_library',
       'dependencies': [
         '../base/base.gyp:base',
         'test_registrar_constants',
@@ skipping 17 matching lines @@
       'include_dirs': [
         '..',
       ],
       'sources': [
         'test/test_registrar_constants.cc',
         'test/test_registrar_constants.h',
       ],
     },
   ],
 }