Use a standard message box for DesktopCastingWarningView.

chrome/browser/ui/simple_message_box.h

Index: chrome/browser/ui/simple_message_box.h
diff --git a/chrome/browser/ui/simple_message_box.h b/chrome/browser/ui/simple_message_box.h
index 4918861fff8a8c737724eaa839542003b6ba471c..1dce6596382cb4b8fe99deb394e276b309b0946c 100644
--- a/chrome/browser/ui/simple_message_box.h
+++ b/chrome/browser/ui/simple_message_box.h
@@ -61,6 +61,10 @@ MessageBoxResult ShowMessageBoxWithButtonText(gfx::NativeWindow parent,
                                               const base::string16& yes_text,
                                               const base::string16& no_text);
 
+// Closes the current message box, if any, accepting or declining based on
+// |accept|. Returns whether there was a message box showing.
+bool CloseMessageBoxForTest(bool accept) WARN_UNUSED_RESULT;
+
 }  // namespace chrome
 
 #endif  // CHROME_BROWSER_UI_SIMPLE_MESSAGE_BOX_H_