Add action for each button on opt-in IME menu.

ui/keyboard/keyboard_controller_observer.h

Index: ui/keyboard/keyboard_controller_observer.h
diff --git a/ui/keyboard/keyboard_controller_observer.h b/ui/keyboard/keyboard_controller_observer.h
index ed80d30141cab7aeed4a038beed4e03dc6a82746..63fa721c1a4023f96d9ed625616e11abc1417802 100644
--- a/ui/keyboard/keyboard_controller_observer.h
+++ b/ui/keyboard/keyboard_controller_observer.h
@@ -24,6 +24,10 @@ class KEYBOARD_EXPORT KeyboardControllerObserver {
 
   // Called when the keyboard was closed.
   virtual void OnKeyboardClosed() = 0;
+
+  // Called when the keyboard has been hidden and the hiding animation finished
+  // successfully
+  virtual void OnKeyboardHidden() {}
 };
 
 }  // namespace keyboard