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..c7585a121c911daedd6f6daf979d5df0cff45508 100644
--- a/chrome/browser/ui/simple_message_box.h
+++ b/chrome/browser/ui/simple_message_box.h
@@ -5,6 +5,7 @@
 #ifndef CHROME_BROWSER_UI_SIMPLE_MESSAGE_BOX_H_
 #define CHROME_BROWSER_UI_SIMPLE_MESSAGE_BOX_H_
 
+#include "base/compiler_specific.h"
 #include "base/strings/string16.h"
 #include "ui/gfx/native_widget_types.h"
 
@@ -61,6 +62,10 @@ 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_