Fixes possible crash in ~Display

services/ui/ws/window_manager_state.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "services/ui/ws/window_manager_state.h"
 
 #include <utility>
 
 #include "base/memory/weak_ptr.h"
 #include "services/shell/public/interfaces/connector.mojom.h"
@@ skipping 101 matching lines @@
 
 WindowManagerState::WindowManagerState(WindowTree* window_tree)
     : window_tree_(window_tree), event_dispatcher_(this), weak_factory_(this) {
   frame_decoration_values_ = mojom::FrameDecorationValues::New();
   frame_decoration_values_->max_title_bar_button_width = 0u;
 
   AddDebugAccelerators();
 }
 
 WindowManagerState::~WindowManagerState() {
+  for (auto& display_root : window_manager_display_roots_)
+    display_root->display()->OnWillDestroyTree(window_tree_);
+
   for (auto& display_root : orphaned_window_manager_display_roots_)
     display_root->root()->RemoveObserver(this);
 }
 
 void WindowManagerState::SetFrameDecorationValues(
     mojom::FrameDecorationValuesPtr values) {
   got_frame_decoration_values_ = true;
   frame_decoration_values_ = values.Clone();
   display_manager()
       ->GetUserDisplayManager(user_id())
@@ skipping 487 matching lines @@
       window->RemoveObserver(this);
       orphaned_window_manager_display_roots_.erase(iter);
       return;
     }
   }
   NOTREACHED();
 }
 
 }  // namespace ws
 }  // namespace ui