Fix Cocoa accessibility callbacks type mismatch

content/browser/accessibility/browser_accessibility_cocoa.mm

 // 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.
 
 #include <execinfo.h>
 
 #import "content/browser/accessibility/browser_accessibility_cocoa.h"
 
 #include <map>
 
@@ skipping 1134 matching lines @@
     subRange = NSMakeRange(index, arrayCount - index);
   } else {
     subRange = NSMakeRange(index, maxCount);
   }
   return [fullArray subarrayWithRange:subRange];
 }
 
 // Returns the count of the specified accessibility array attribute.
 - (NSUInteger)accessibilityArrayAttributeCount:(NSString*)attribute {
   if (!browserAccessibility_)
-    return nil;
+    return 0;
 
   NSArray* fullArray = [self accessibilityAttributeValue:attribute];
   return [fullArray count];
 }
 
 // Returns the list of accessibility attributes that this object supports.
 - (NSArray*)accessibilityAttributeNames {
   if (!browserAccessibility_)
     return nil;
 
@@ skipping 141 matching lines @@
   }
   // TODO(aboxhall): expose NSAccessibilityServesAsTitleForUIElementsAttribute
   // for elements which are referred to by labelledby or are labels
 
   return ret;
 }
 
 // Returns the index of the child in this objects array of children.
 - (NSUInteger)accessibilityGetIndexOf:(id)child {
   if (!browserAccessibility_)
-    return nil;
+    return 0;
 
   NSUInteger index = 0;
   for (BrowserAccessibilityCocoa* childToCheck in [self children]) {
     if ([child isEqual:childToCheck])
       return index;
     ++index;
   }
   return NSNotFound;
 }
 
 // Returns whether or not the specified attribute can be set by the
 // accessibility API via |accessibilitySetValue:forAttribute:|.
 - (BOOL)accessibilityIsAttributeSettable:(NSString*)attribute {
   if (!browserAccessibility_)
-    return nil;
+    return NO;
 
   if ([attribute isEqualToString:NSAccessibilityFocusedAttribute])
     return GetState(browserAccessibility_,
         ui::AX_STATE_FOCUSABLE);
   if ([attribute isEqualToString:NSAccessibilityValueAttribute]) {
     return browserAccessibility_->GetBoolAttribute(
         ui::AX_ATTR_CAN_SET_VALUE);
   }
   if ([attribute isEqualToString:NSAccessibilitySelectedTextRangeAttribute] &&
       ([[self role] isEqualToString:NSAccessibilityTextFieldRole] ||
@@ skipping 108 matching lines @@
     return [super hash];
   return browserAccessibility_->GetId();
 }
 
 - (BOOL)accessibilityShouldUseUniqueId {
   return YES;
 }
 
 @end