Revert of Do not activate TrayBubbleView by default

ash/system/tray/system_tray.h

Index: ash/system/tray/system_tray.h
diff --git a/ash/system/tray/system_tray.h b/ash/system/tray/system_tray.h
index 8483db9220a9666d30524d588ae785b695d249d9..fa16c53f74067aaee8e1eacaccff91add8d1d17d 100644
--- a/ash/system/tray/system_tray.h
+++ b/ash/system/tray/system_tray.h
@@ -18,6 +18,7 @@
 
 namespace ash {
 
+class KeyEventWatcher;
 enum class LoginStatus;
 class ScreenTrayItem;
 class SystemBubbleWrapper;
@@ -132,12 +133,7 @@
   void BubbleViewDestroyed() override;
   void OnMouseEnteredView() override;
   void OnMouseExitedView() override;
-  void RegisterAccelerators(const std::vector<ui::Accelerator>& accelerators,
-                            views::TrayBubbleView* tray_bubble_view) override;
-  void UnregisterAllAccelerators(
-      views::TrayBubbleView* tray_bubble_view) override;
   base::string16 GetAccessibleNameForBubble() override;
-  bool ShouldEnableExtraKeyboardAccessibility() override;
   void HideBubble(const views::TrayBubbleView* bubble_view) override;
 
   ScreenTrayItem* GetScreenShareItem() { return screen_share_tray_item_; }
@@ -155,6 +151,10 @@
 
   // Activates the bubble and starts key navigation with the |key_event|.
   void ActivateAndStartNavigation(const ui::KeyEvent& key_event);
+
+  // Creates the key event watcher. See |ShowItems()| for why key events are
+  // observed.
+  void CreateKeyEventWatcher();
 
   // Creates the default set of items for the sytem tray.
   void CreateItems(SystemTrayDelegate* delegate);
@@ -232,6 +232,8 @@
   ScreenTrayItem* screen_capture_tray_item_ = nullptr;  // not owned
   ScreenTrayItem* screen_share_tray_item_ = nullptr;    // not owned
 
+  std::unique_ptr<KeyEventWatcher> key_event_watcher_;
+
   std::unique_ptr<ActivationObserver> activation_observer_;
 
   DISALLOW_COPY_AND_ASSIGN(SystemTray);