[Windows] Finish global and non-global media keys support on Windows.

chrome/browser/extensions/global_shortcut_listener_win.cc

 // Copyright (c) 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.
 
 #include "chrome/browser/extensions/global_shortcut_listener_win.h"
 
 #include "base/win/win_util.h"
 #include "content/public/browser/browser_thread.h"
 #include "ui/base/accelerators/accelerator.h"
 #include "ui/events/event_constants.h"
@@ skipping 54 matching lines @@
   modifiers |= (LOWORD(lparam) & MOD_CONTROL) ? ui::EF_CONTROL_DOWN : 0;
   ui::Accelerator accelerator(
       ui::KeyboardCodeForWindowsKeyCode(key_code), modifiers);
 
   instance.Get().NotifyKeyPressed(accelerator);
 }
 
 void GlobalShortcutListenerWin::RegisterAccelerator(
     const ui::Accelerator& accelerator,
     GlobalShortcutListener::Observer* observer) {
+  if (hotkey_ids_.find(accelerator) != hotkey_ids_.end()) {
+    // The shortcut has already been registered. Some shortcuts, such as
+    // MediaKeys can have multiple targets, all keyed off of the same
+    // accelerator.
+    return;
+  }
+
   int modifiers = 0;
   modifiers |= accelerator.IsShiftDown() ? MOD_SHIFT : 0;
   modifiers |= accelerator.IsCtrlDown() ? MOD_CONTROL : 0;
   modifiers |= accelerator.IsAltDown() ? MOD_ALT : 0;
   static int hotkey_id = 0;
   bool success = !!RegisterHotKey(
       gfx::SingletonHwnd::GetInstance()->hwnd(),
       hotkey_id,
       modifiers,
       accelerator.key_code());
@@ skipping 22 matching lines @@
       gfx::SingletonHwnd::GetInstance()->hwnd(), it->second);
   // This call should always succeed, as long as we pass in the right HWND and
   // an id we've used to register before.
   DCHECK(success);
 
   hotkey_ids_.erase(it);
   GlobalShortcutListener::UnregisterAccelerator(accelerator, observer);
 }
 
 }  // namespace extensions