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

chrome/browser/extensions/global_shortcut_listener_win.cc

Index: chrome/browser/extensions/global_shortcut_listener_win.cc
diff --git a/chrome/browser/extensions/global_shortcut_listener_win.cc b/chrome/browser/extensions/global_shortcut_listener_win.cc
index a5987f7ab9eca678c502dd225cbfb78170a760dc..318e06e11a3647f6d13c76995f0f104c3406a340 100644
--- a/chrome/browser/extensions/global_shortcut_listener_win.cc
+++ b/chrome/browser/extensions/global_shortcut_listener_win.cc
@@ -72,6 +72,13 @@ void GlobalShortcutListenerWin::OnWndProc(HWND hwnd,
 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;