Fix Cocoa accessibility callbacks type mismatch

content/browser/accessibility/browser_accessibility_cocoa.mm

Index: content/browser/accessibility/browser_accessibility_cocoa.mm
diff --git a/content/browser/accessibility/browser_accessibility_cocoa.mm b/content/browser/accessibility/browser_accessibility_cocoa.mm
index 5086982a90f464783dbc36c5e8fac41c19e7acbb..a4b31fc227b19ee2e5d79a32dc0bab41b6942e3d 100644
--- a/content/browser/accessibility/browser_accessibility_cocoa.mm
+++ b/content/browser/accessibility/browser_accessibility_cocoa.mm
@@ -1152,7 +1152,7 @@ NSDictionary* attributeToMethodNameMap = nil;
 // 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];
@@ -1314,7 +1314,7 @@ NSDictionary* attributeToMethodNameMap = nil;
 // 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]) {
@@ -1329,7 +1329,7 @@ NSDictionary* attributeToMethodNameMap = nil;
 // accessibility API via |accessibilitySetValue:forAttribute:|.
 - (BOOL)accessibilityIsAttributeSettable:(NSString*)attribute {
   if (!browserAccessibility_)
-    return nil;
+    return NO;
 
   if ([attribute isEqualToString:NSAccessibilityFocusedAttribute])
     return GetState(browserAccessibility_,