Mac: fix Esc-key cancellation of bookmark bar button drags.

chrome/browser/cocoa/bookmark_button.h

 // Copyright (c) 2009 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>
 
 // Class for bookmark bar buttons that can be drag sources.
 @interface BookmarkButton : NSButton {
  @private
-  BOOL draggable_;
-  BOOL beingDragged_;  // are we being dragged?
+  BOOL draggable_;     // Is this a draggable type of button?
+  BOOL mayDragStart_;  // Set to YES on mouse down, NO on up or drag.
+  BOOL beingDragged_;
 
   // Initial mouse-down to prevent a hair-trigger drag.
   NSPoint initialMouseDownLocation_;
 }
 
 // Enable or disable dragability for special buttons like "Other Bookmarks".
 @property BOOL draggable;
 
 @end
 
 extern NSString* kBookmarkButtonDragType;