Makes ScreenDimmer deal with the shell going away

ash/common/wm/screen_dimmer.cc

 // Copyright (c) 2012 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 "ash/common/wm/screen_dimmer.h"
 
 #include "ash/common/shell_window_ids.h"
 #include "ash/common/wm/container_finder.h"
 #include "ash/common/wm/window_dimmer.h"
 #include "ash/common/wm_shell.h"
@@ skipping 14 matching lines @@
 
 ScreenDimmer::ScreenDimmer(Container container)
     : container_(container),
       is_dimming_(false),
       at_bottom_(false),
       window_dimmers_(base::MakeUnique<WmWindowUserData<WindowDimmer>>()) {
   WmShell::Get()->AddShellObserver(this);
 }
 
 ScreenDimmer::~ScreenDimmer() {
-  WmShell::Get()->RemoveShellObserver(this);
+  // Usage in chrome results in ScreenDimmer outliving the shell.
+  if (WmShell::HasInstance())

[James Cook, 2016/09/20 23:26:18]

Does this mean we'll never clean up this observer in classic ash?

[sky, 2016/09/20 23:33:05]

That is correct. The ObserverList does not enforce it is empty on destruction. 

+    WmShell::Get()->RemoveShellObserver(this);
 }
 
 void ScreenDimmer::SetDimming(bool should_dim) {
   if (should_dim == is_dimming_)
     return;
   is_dimming_ = should_dim;
 
   Update(should_dim);
 }
 
@@ skipping 25 matching lines @@
       else
         container->StackChildAtTop(window_dimmer->window());
       window_dimmer->window()->Show();
     } else if (window_dimmer) {
       window_dimmers_->Set(container, nullptr);
     }
   }
 }
 
 }  // namespace ash