Fix crash when system retains an accessibility object.

content/browser/accessibility/browser_accessibility_cocoa.h

Index: content/browser/accessibility/browser_accessibility_cocoa.h
diff --git a/content/browser/accessibility/browser_accessibility_cocoa.h b/content/browser/accessibility/browser_accessibility_cocoa.h
index 0d401a0332eaf28857916c81a90a432a66185842..387c5b2938b545121a464a410bdd6ac6cbd4652e 100644
--- a/content/browser/accessibility/browser_accessibility_cocoa.h
+++ b/content/browser/accessibility/browser_accessibility_cocoa.h
@@ -30,6 +30,11 @@
 - (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;