[Mac] Add a confirm to quit experiment to about:flags.

chrome/browser/cocoa/confirm_quit_panel_controller.h

+// Copyright (c) 2010 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#import <Cocoa/Cocoa.h>
+
+#include "base/cocoa_protocols_mac.h"
+
+// The ConfirmQuitPanelController manages the black HUD window that tells users
+// to "Hold Cmd+Q to Quit".
+@interface ConfirmQuitPanelController : NSWindowController<NSWindowDelegate> {
+}
+
+// Designated initializer. Loads window from NIB but does not show it.
+- (id)init;
+
+// Shows the window.
+- (void)showWindow:(id)sender;
+
+// If the user did not confirm quit, send this message to give the user
+// instructions on how to quit.
+- (void)dismissPanel;
+
+@end