Add a11y on-screen keyboard to accessibility menu on system tray.

ash/system/tray_accessibility.h

Index: ash/system/tray_accessibility.h
diff --git a/ash/system/tray_accessibility.h b/ash/system/tray_accessibility.h
index a3f53cbfec276d5bbddd9ce71dc71f8d8e3985bf..21396e57327dea4c1a340ca45aa99fc123505e59 100644
--- a/ash/system/tray_accessibility.h
+++ b/ash/system/tray_accessibility.h
@@ -77,12 +77,14 @@ class AccessibilityDetailedView : public TrayDetailsView,
   views::View* help_view_;
   views::View* settings_view_;
   views::View* autoclick_view_;
+  views::View* virtual_keyboard_view_;
 
   bool spoken_feedback_enabled_;
   bool high_contrast_enabled_;
   bool screen_magnifier_enabled_;
   bool large_cursor_enabled_;
   bool autoclick_enabled_;
+  bool virtual_keyboard_enabled_;
   user::LoginStatus login_;
 
   friend class chromeos::TrayAccessibilityTest;