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 ebc7afc7762ff4d5ddc30329dd319c65a7df032d..65b016762975a70a5260753bb5c6469cefa5028f 100644
--- a/ash/system/tray/system_tray.h
+++ b/ash/system/tray/system_tray.h
@@ -132,7 +132,12 @@ class ASH_EXPORT SystemTray : public TrayBackgroundView,
   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_; }
@@ -241,8 +246,6 @@ class ASH_EXPORT SystemTray : public TrayBackgroundView,
   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);