Have browser process shutdown metro viewer on Metro -> Metro restart.

ui/metro_viewer/metro_viewer_messages.h

 // Copyright 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.
 
 // Multiply-included message file, no include guard.
 
 #include <vector>
 
 #include "base/basictypes.h"
 #include "ipc/ipc_message_macros.h"
@@ skipping 87 matching lines @@
                      bool,           /* success */
                      base::FilePath) /* filepath*/
 
 // Messages sent from the browser to the viewer:
 
 // Requests the viewer to activate desktop mode.
 IPC_MESSAGE_CONTROL2(MetroViewerHostMsg_ActivateDesktop,
                      base::FilePath /* shortcut */,
                      bool           /* ash exit */);
 
+// Request the viewer to close itself gracefully.
+IPC_MESSAGE_CONTROL0(MetroViewerHostMsg_MetroExit);
+
 // Requests the viewer to open a URL in desktop mode.
 IPC_MESSAGE_CONTROL2(MetroViewerHostMsg_OpenURLOnDesktop,
                      base::FilePath,  /* shortcut */
                      base::string16         /* url */);
 
 // Requests the viewer to change the pointer to a new cursor.
 IPC_MESSAGE_CONTROL1(MetroViewerHostMsg_SetCursor,
                      int64         /* cursor */);
 
 // This structure contains the parameters sent to the viewer process to display
@@ skipping 93 matching lines @@
   IPC_STRUCT_TRAITS_MEMBER(right)
   IPC_STRUCT_TRAITS_MEMBER(bottom)
 IPC_STRUCT_TRAITS_END()
 
 // Requests the viewer to update the document context such as attached
 // InputScopes and character bounds.
 IPC_MESSAGE_CONTROL2(
     MetroViewerHostMsg_ImeTextInputClientUpdated,
     std::vector<int32>,                           /* InputScope enums */
     std::vector<metro_viewer::CharacterBounds>)   /* character bounds */