Handle UNPIN operation in ash.

ash/wm/screen_pinning_controller.h

Index: ash/wm/screen_pinning_controller.h
diff --git a/ash/wm/screen_pinning_controller.h b/ash/wm/screen_pinning_controller.h
index 0e0b42d861259b816a144ad0cf54f5e076667575..191ac0828f0281e5b03aedbc8a4e8ad87b3e93e1 100644
--- a/ash/wm/screen_pinning_controller.h
+++ b/ash/wm/screen_pinning_controller.h
@@ -31,6 +31,9 @@ class ScreenPinningController : public WindowTreeHostManager::Observer {
   // Returns true if in pinned mode, otherwise false.
   bool IsPinned() const;
 
+  // Returns the pinned window if in pinned mode, or nullptr.
+  WmWindow* pinned_window() const { return pinned_window_; }
+
   // Called when a new window is added to the container which has the pinned
   // window.
   void OnWindowAddedToPinnedContainer(WmWindow* new_window);