Do not return an AXValue for buttons to match Safari....

chrome/browser/accessibility/browser_accessibility_cocoa.mm

 // Copyright (c) 2010 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 "chrome/browser/accessibility/browser_accessibility_cocoa.h"
 
 #include "app/l10n_util_mac.h"
 #include "base/string16.h"
@@ skipping 192 matching lines @@
   }
   if ([attribute isEqualToString:NSAccessibilityTitleAttribute]) {
     return base::SysUTF16ToNSString(browserAccessibility_->name());
   }
   if ([attribute isEqualToString:NSAccessibilityHelpAttribute]) {
     return NSStringForWebAccessibilityAttribute(
         browserAccessibility_->attributes(),
         WebAccessibility::ATTR_HELP);
   }
   if ([attribute isEqualToString:NSAccessibilityValueAttribute]) {
+    // WebCore uses an attachmentView to get the below behavior.
+    // We do not have any native views backing this object, so need
+    // to approximate Cocoa ax behavior best as we can.
     if ([self role] == @"AXHeading") {
       NSString* headingLevel =
           NSStringForWebAccessibilityAttribute(
               browserAccessibility_->attributes(),
               WebAccessibility::ATTR_HTML_TAG);
       if ([headingLevel length] >= 2) {
         return [NSNumber numberWithInt:
             [[headingLevel substringFromIndex:1] intValue]];
       }
+    } else if ([self role] == NSAccessibilityButtonRole) {
+      // AXValue does not make sense for pure buttons.
+      return @"";
     } else if ([self role] == NSAccessibilityCheckBoxRole) {
       return [NSNumber numberWithInt:GetState(
           browserAccessibility_, WebAccessibility::STATE_CHECKED) ? 1 : 0];
     } else {
       return base::SysUTF16ToNSString(browserAccessibility_->value());
     }
   }
   if ([attribute isEqualToString:NSAccessibilityRoleDescriptionAttribute]) {
     return [self roleDescription];
   }
@@ skipping 228 matching lines @@
 
 - (NSUInteger)hash {
   // Potentially called during dealloc.
   if (!browserAccessibility_)
     return [super hash];
   return browserAccessibility_->renderer_id();
 }
 
 @end