Use a standard message box for DesktopCastingWarningView.

components/constrained_window/constrained_window_views.cc

Index: components/constrained_window/constrained_window_views.cc
diff --git a/components/constrained_window/constrained_window_views.cc b/components/constrained_window/constrained_window_views.cc
index ccdfdd1606700fa05b4f4d4cfdd5ab52e98489c2..fa581988e9ba8ea105eee3f28c1b3af82065f053 100644
--- a/components/constrained_window/constrained_window_views.cc
+++ b/components/constrained_window/constrained_window_views.cc
@@ -201,7 +201,8 @@ views::Widget* CreateBrowserModalDialogViews(views::DialogDelegate* dialog,
   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;
@@ -219,8 +220,9 @@ views::Widget* CreateBrowserModalDialogViews(views::DialogDelegate* dialog,
   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 =