[Mac] Update tab close button when user drags outside

ui/base/cocoa/hover_button.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 UI_BASE_COCOA_HOVER_BUTTON_
 #define UI_BASE_COCOA_HOVER_BUTTON_
 
 #import <Cocoa/Cocoa.h>
 
 #import "ui/base/cocoa/tracking_area.h"
 #import "ui/base/ui_base_export.h"
 
 // A button that changes when you hover over it and click it.
 UI_BASE_EXPORT
 @interface HoverButton : NSButton {
  @protected
   // Enumeration of the hover states that the close button can be in at any one
   // time. The button cannot be in more than one hover state at a time.
   enum HoverState {
     kHoverStateNone = 0,
     kHoverStateMouseOver = 1,
     kHoverStateMouseDown = 2
   };
 
   HoverState hoverState_;
 
  @private
   // Tracking area for button mouseover states. Nil if not enabled.
   ui::ScopedCrTrackingArea trackingArea_;
+  BOOL mouseDown_;
 }
 
 @property(nonatomic) HoverState hoverState;
 
+// Common initialization called from initWithFrame: and awakeFromNib.
+// Subclassers should call [super commonInit].
+- (void)commonInit;
+
 // Text that would be announced by screen readers.
 - (void)setAccessibilityTitle:(NSString*)accessibilityTitle;
 
 // Enables or disables the tracking for the button.
 - (void)setTrackingEnabled:(BOOL)enabled;
 
 // Checks to see whether the mouse is in the button's bounds and update
 // the image in case it gets out of sync.  This occurs to the close button
 // when you close a tab so the tab to the left of it takes its place, and
 // drag the button without moving the mouse before you press the button down.
 - (void)checkImageState;
 
 @end
 
 #endif  // UI_BASE_COCOA_HOVER_BUTTON_