[Mac] Create CrTrackingArea and use it in TabStripController.

chrome/browser/ui/cocoa/tracking_area.h

+// Copyright (c) 2011 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.
+
+#ifndef CHROME_BROWSER_UI_COCOA_TRACKING_AREA_H_
+#define CHROME_BROWSER_UI_COCOA_TRACKING_AREA_H_
+
+#import <AppKit/AppKit.h>
+
+#include "base/scoped_nsobject.h"
+
+@class CrTrackingAreaOwnerProxy;
+
+// The CrTrackingArea can be used in place of an NSTrackingArea to shut off
+// messaging to the |owner| at a specific point in time.
+@interface CrTrackingArea : NSTrackingArea {
+ @private
+  scoped_nsobject<CrTrackingAreaOwnerProxy> ownerProxy_;
+}
+
+// Designated initializer. Forwards all arguments to the superclass, but wraps
+// |owner| in a proxy object.
+- (id)initWithRect:(NSRect)rect
+           options:(NSTrackingAreaOptions)options
+             owner:(id)owner
+          userInfo:(NSDictionary*)userInfo;
+
+// Prevents any future messages from being delivered to the |owner|.
+- (void)clearOwner;
+
+// Watches |window| for its NSWindowWillCloseNotification and calls
+// |-clearOwner| when the notification is observed.
+- (void)clearOwnerWhenWindowWillClose:(NSWindow*)window;
+
+@end
+
+#endif  // CHROME_BROWSER_UI_COCOA_TRACKING_AREA_H_