Adds the UserInputMonitor implementation for Mac.

media/base/user_input_monitor.cc

Index: media/base/user_input_monitor.cc
diff --git a/media/base/user_input_monitor.cc b/media/base/user_input_monitor.cc
index 18b4c8060f64e34437d4e3e9222fde4d18af520a..b1005538e309d4b3a335078eb3b14c58c9c812fc 100644
--- a/media/base/user_input_monitor.cc
+++ b/media/base/user_input_monitor.cc
@@ -71,13 +71,25 @@ void UserInputMonitor::OnKeyboardEvent(ui::EventType event,
                                        ui::KeyboardCode key_code) {
   base::AutoLock auto_lock(lock_);
   // Updates the pressed keys and maybe notifies the key_stroke_listeners_.
+  // The Mac implementation does not listen to the the key down/up events, but
+  // polls the key state and only notifies UserInputMonitor when a key is down
+  // now but not in the previous polling. So we do not check/update
+  // |pressed_keys_| here.
   if (event == ui::ET_KEY_PRESSED) {
+#if !defined(OS_MACOSX)
     if (pressed_keys_.find(key_code) != pressed_keys_.end())
       return;
     pressed_keys_.insert(key_code);
+#endif
+
     DVLOG(6) << "Key stroke detected.";
     FOR_EACH_OBSERVER(KeyStrokeListener, key_stroke_listeners_, OnKeyStroke());
   } else {
+#if defined(OS_MACOSX)
+    NOTREACHED();
+    return;
+#endif
+
     DCHECK_EQ(ui::ET_KEY_RELEASED, event);
     DCHECK(pressed_keys_.find(key_code) != pressed_keys_.end());
     pressed_keys_.erase(key_code);