Use a standard message box for DesktopCastingWarningView.

chrome/browser/ui/cocoa/simple_message_box_mac.mm

Index: chrome/browser/ui/cocoa/simple_message_box_mac.mm
diff --git a/chrome/browser/ui/cocoa/simple_message_box_mac.mm b/chrome/browser/ui/cocoa/simple_message_box_mac.mm
index 5a14b017cac2334a987ca5379c32ce3bef53f050..7c71c80b43b937cebb722c11d3ac4936246ae475 100644
--- a/chrome/browser/ui/cocoa/simple_message_box_mac.mm
+++ b/chrome/browser/ui/cocoa/simple_message_box_mac.mm
@@ -84,4 +84,9 @@ MessageBoxResult ShowQuestionMessageBox(gfx::NativeWindow parent,
                         MESSAGE_BOX_TYPE_QUESTION);
 }
 
+bool CloseMessageBoxForTest(bool accept) {
+  NOTIMPLEMENTED();
+  return false;
+}
+
 }  // namespace chrome