Fix crash when system retains an accessibility object.

content/browser/accessibility/browser_accessibility_cocoa.h

 // Copyright (c) 2012 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 CONTENT_BROWSER_ACCESSIBILITY_BROWSER_ACCESSIBILITY_COCOA_H_
 #define CONTENT_BROWSER_ACCESSIBILITY_BROWSER_ACCESSIBILITY_COCOA_H_
 
 #import <Cocoa/Cocoa.h>
 
 #import "base/memory/scoped_nsobject.h"
@@ skipping 12 matching lines @@
   id<BrowserAccessibilityDelegateCocoa> delegate_;
 }
 
 // This creates a cocoa browser accessibility object around
 // the cross platform BrowserAccessibility object.  The delegate is
 // used to communicate with the host renderer.  None of these
 // parameters can be null.
 - (id)initWithObject:(content::BrowserAccessibility*)accessibility
             delegate:(id<BrowserAccessibilityDelegateCocoa>)delegate;
 
+// Clear this object's pointer to the wrapped BrowserAccessibility object
+// because the wrapped object has been deleted, but this object may
+// persist if the system still has references to it.
+- (void)detach;
+
 // Invalidate children for a non-ignored ancestor (including self).
 - (void)childrenChanged;
 
 // Convenience method to get the internal, cross-platform role
 // from browserAccessibility_.
 - (content::AccessibilityNodeData::Role)internalRole;
 
 // Return the method name for the given attribute. For testing only.
 - (NSString*)methodNameForAttribute:(NSString*)attribute;
 
@@ skipping 55 matching lines @@
 @property(nonatomic, readonly) NSString* valueDescription;
 @property(nonatomic, readonly) NSValue* visibleCharacterRange;
 @property(nonatomic, readonly) NSArray* visibleCells;
 @property(nonatomic, readonly) NSArray* visibleColumns;
 @property(nonatomic, readonly) NSArray* visibleRows;
 @property(nonatomic, readonly) NSNumber* visited;
 @property(nonatomic, readonly) id window;
 @end
 
 #endif // CONTENT_BROWSER_ACCESSIBILITY_BROWSER_ACCESSIBILITY_COCOA_H_