Use a standard message box for DesktopCastingWarningView.

chrome/browser/ui/views/simple_message_box_views.cc

Index: chrome/browser/ui/views/simple_message_box_views.cc
diff --git a/chrome/browser/ui/views/simple_message_box_views.cc b/chrome/browser/ui/views/simple_message_box_views.cc
index 4eee13d9bbbd578f364f10bd44db81ef41033ce1..91c219394eff2a5b098cd8d88d6deecae3c9b67b 100644
--- a/chrome/browser/ui/views/simple_message_box_views.cc
+++ b/chrome/browser/ui/views/simple_message_box_views.cc
@@ -73,6 +73,9 @@ class SimpleMessageBoxViews : public views::DialogDelegate {
   DISALLOW_COPY_AND_ASSIGN(SimpleMessageBoxViews);
 };
 
+// The currently showing message box, if there is one. Used for tests.
+SimpleMessageBoxViews* g_current_message_box = nullptr;
+
 ////////////////////////////////////////////////////////////////////////////////
 // SimpleMessageBoxViews, public:
 
@@ -112,6 +115,7 @@ SimpleMessageBoxViews::~SimpleMessageBoxViews() {
 }
 
 MessageBoxResult SimpleMessageBoxViews::RunDialogAndGetResult() {
+  g_current_message_box = this;

[msw, 2016/10/27 21:46:45]

Should this care if there's any other dialog already showing?

[Evan Stade, 2016/10/28 17:31:55]

Given how it's presently used (only for testing), I guess not? Do you think it
should?

   MessageBoxResult result = MESSAGE_BOX_RESULT_NO;
   result_ = &result;
   // TODO(pkotwicz): Exit message loop when the dialog is closed by some other
@@ -121,6 +125,7 @@ MessageBoxResult SimpleMessageBoxViews::RunDialogAndGetResult() {
   base::RunLoop run_loop;
   quit_runloop_ = run_loop.QuitClosure();
   run_loop.Run();
+  g_current_message_box = nullptr;
   return result;
 }
 
@@ -247,6 +252,17 @@ MessageBoxResult ShowMessageBoxImpl(gfx::NativeWindow parent,
 
 }  // namespace
 
+bool CloseMessageBoxForTest(bool accept) {
+  if (!g_current_message_box)
+    return false;
+
+  if (accept)
+    g_current_message_box->Accept();
+  else
+    g_current_message_box->Cancel();
+  return true;
+}
+
 void ShowWarningMessageBox(gfx::NativeWindow parent,
                            const base::string16& title,
                            const base::string16& message) {