ProcessSingleton now uses base::win::MessageWindow to create a message-only window.

base/win/message_window.h

 // Copyright 2013 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_WIN_MESSAGE_WINDOW_H_
 #define BASE_WIN_MESSAGE_WINDOW_H_
 
 #include <windows.h>
 
 #include "base/base_export.h"
 #include "base/basictypes.h"
 #include "base/callback.h"
 #include "base/compiler_specific.h"
 #include "base/strings/string16.h"
 #include "base/threading/non_thread_safe.h"
 
 namespace base {
 namespace win {
 
 // Implements a message-only window.
 class BASE_EXPORT MessageWindow : public base::NonThreadSafe {
  public:
+  // Used to register a process-wide message window class.
+  class WindowClass;
+
   // Implement this callback to handle messages received by the message window.
   // If the callback returns |false|, the first four parameters are passed to
   // DefWindowProc(). Otherwise, |*result| is returned by the window procedure.
   typedef base::Callback<bool(UINT message,
                               WPARAM wparam,
                               LPARAM lparam,
                               LRESULT* result)> MessageCallback;
 
   MessageWindow();
   ~MessageWindow();
 
   // Creates a message-only window. The incoming messages will be passed by
   // |message_callback|. |message_callback| must outlive |this|.
   bool Create(const MessageCallback& message_callback);
 
   // Same as Create() but assigns the name to the created window.
   bool CreateNamed(const MessageCallback& message_callback,
                    const string16& window_name);
 
   HWND hwnd() const { return window_; }
 
+  // Retrieves a handle of the first message-only window with matching
+  // |windows_name|.
+  static HWND FindWindow(const string16& window_name);
+
  private:
+  // Give |WindowClass| access to WindowProc().
+  friend class WindowClass;
+
   // Contains the actual window creation code.
   bool DoCreate(const MessageCallback& message_callback,
                 const wchar_t* window_name);
 
   // Invoked by the OS to process incoming window messages.
   static LRESULT CALLBACK WindowProc(HWND hwnd, UINT message, WPARAM wparam,
                                      LPARAM lparam);
 
-  // Atom representing the registered window class.
-  ATOM atom_;
-
   // Invoked to handle messages received by the window.
   MessageCallback message_callback_;
 
   // Handle of the input window.
   HWND window_;
 
   DISALLOW_COPY_AND_ASSIGN(MessageWindow);
 };
 
 }  // namespace win
 }  // namespace base
 
 #endif  // BASE_WIN_MESSAGE_WINDOW_H_