Have WindowTreeClient detect error state earlier

ui/views/mus/pointer_watcher_event_router.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 "ui/views/mus/pointer_watcher_event_router.h"
 
 #include "ui/aura/client/capture_client.h"
 #include "ui/aura/mus/window_tree_client.h"
 #include "ui/aura/window.h"
 #include "ui/display/screen.h"
@@ skipping 145 matching lines @@
                                    gfx::Point(), ui::EventTimeForNow(), 0, 0);
   const ui::PointerEvent event(mouse_event);
   gfx::Point location_in_screen =
       display::Screen::GetScreen()->GetCursorScreenPoint();
   for (PointerWatcher& observer : move_watchers_)
     observer.OnPointerEventObserved(event, location_in_screen, nullptr);
   for (PointerWatcher& observer : non_move_watchers_)
     observer.OnPointerEventObserved(event, location_in_screen, nullptr);
 }
 
-void PointerWatcherEventRouter::OnDidDestroyClient(
+void PointerWatcherEventRouter::OnWillDestroyClient(
     aura::WindowTreeClient* client) {
   // We expect that all observers have been removed by this time.
   DCHECK_EQ(event_types_, EventTypes::NONE);
   DCHECK_EQ(client, window_tree_client_);
   window_tree_client_->RemoveObserver(this);
   window_tree_client_ = nullptr;
 }
 
 }  // namespace views