Use a standard message box for DesktopCastingWarningView.

components/constrained_window/constrained_window_views.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 "components/constrained_window/constrained_window_views.h"
 
 #include <algorithm>
 
 #include "base/macros.h"
 #include "build/build_config.h"
@@ skipping 183 matching lines @@
           ->delegate()
           ->GetWebContentsModalDialogHost()
           ->GetHostView());
 }
 
 views::Widget* CreateBrowserModalDialogViews(views::DialogDelegate* dialog,
                                              gfx::NativeWindow parent) {
   DCHECK_NE(ui::MODAL_TYPE_CHILD, dialog->GetModalType());
   DCHECK_NE(ui::MODAL_TYPE_NONE, dialog->GetModalType());
 
-  DCHECK(constrained_window_views_client);
+  if (parent)
+    DCHECK(constrained_window_views_client);
   gfx::NativeView parent_view =
       parent ? constrained_window_views_client->GetDialogHostView(parent)
              : nullptr;
   views::Widget* widget =
       views::DialogDelegate::CreateDialogWidget(dialog, NULL, parent_view);
 
   bool requires_positioning = dialog->ShouldUseCustomFrame();
 
 #if defined(OS_MACOSX)
   // On Mac, window modal dialogs are displayed as sheets, so their position is
   // managed by the parent window.
   requires_positioning = false;
 #endif
 
   if (!requires_positioning)
     return widget;
 
-  ModalDialogHost* host = constrained_window_views_client->
-      GetModalDialogHost(parent);
+  ModalDialogHost* host =
+      parent ? constrained_window_views_client->GetModalDialogHost(parent)

[Evan Stade, 2016/10/27 16:36:47]

note that this is not a behavioral change as the call already returns null
output for null input --- this just short circuits the lack of a
constrained_window_views_client

+             : nullptr;
   if (host) {
     DCHECK_EQ(parent_view, host->GetHostView());
     ModalDialogHostObserver* dialog_host_observer =
         new WidgetModalDialogHostObserverViews(
             host, widget, kWidgetModalDialogHostObserverViewsKey);
     dialog_host_observer->OnPositionRequiresUpdate();
   }
   return widget;
 }
 
 }  // namespace constrained window