Fix accessibility constant clash in OSX 10.12.

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>
 #include <stddef.h>
 #include <stdint.h>
 
 #import "content/browser/accessibility/browser_accessibility_cocoa.h"
 
@@ skipping 35 matching lines @@
 NSString* const NSAccessibilityARIAPosInSetAttribute = @"AXARIAPosInSet";
 NSString* const NSAccessibilityARIARelevantAttribute = @"AXARIARelevant";
 NSString* const NSAccessibilityARIASetSizeAttribute = @"AXARIASetSize";
 NSString* const NSAccessibilityAccessKeyAttribute = @"AXAccessKey";
 NSString* const NSAccessibilityDropEffectsAttribute = @"AXDropEffects";
 NSString* const NSAccessibilityGrabbedAttribute = @"AXGrabbed";
 NSString* const NSAccessibilityInvalidAttribute = @"AXInvalid";
 NSString* const NSAccessibilityIsMultiSelectableAttribute =
     @"AXIsMultiSelectable";
 NSString* const NSAccessibilityLoadingProgressAttribute = @"AXLoadingProgress";
+#if !defined(MAC_OS_X_VERSION_10_12) || \
+    MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_X_VERSION_10_12

[pwnall, 2016/08/19 08:15:03]

Using MAC_OS_X_VERSION_MIN_REQUIRED doesn't actually work, because we retain the
same minimum version when compiling against the 10.12 SDK as before.
MAC_OS_X_VERSION_MAX_ALLOWED (also used in base/mac/sdk_forward_declarations.h)
matches the desired intent to get the block excluded when compiling against SDKs
>= 10.12.

Unfortunately, the SDK definition is marked  NS_AVAILABLE_MAC(10_12), meaning
that AppKit does not actually export the symbol before OSX 10.12. This triggers
the following warning, which breaks the build.

../../content/browser/accessibility/browser_accessibility_cocoa.mm:556:8: error:
'NSAccessibilityRequiredAttribute' is partial: introduced in macOS 10.12
[-Werror,-Wpartial-availability]
      {NSAccessibilityRequiredAttribute, @"required"},
       ^
/Applications/Xcode-beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk/System/Library/Frameworks/AppKit.framework/Headers/NSAccessibilityConstants.h:89:31:
note: 'NSAccessibilityRequiredAttribute' has been explicitly marked partial here
APPKIT_EXTERN NSString *const NSAccessibilityRequiredAttribute
NS_AVAILABLE_MAC(10_12);  //(NSNumber *) - (boolValue) whether a form field is
required to have content for successful submission of the form
                              ^
../../content/browser/accessibility/browser_accessibility_cocoa.mm:556:8: note:
explicitly redeclare 'NSAccessibilityRequiredAttribute' to silence this warning
      {NSAccessibilityRequiredAttribute, @"required"},

 NSString* const NSAccessibilityRequiredAttribute = @"AXRequired";
+#endif
 NSString* const
     NSAccessibilityUIElementCountForSearchPredicateParameterizedAttribute =
         @"AXUIElementCountForSearchPredicate";
 NSString* const
     NSAccessibilityUIElementsForSearchPredicateParameterizedAttribute =
         @"AXUIElementsForSearchPredicate";
 NSString* const NSAccessibilityVisitedAttribute = @"AXVisited";
 
 // Private attributes for text markers.
 NSString* const NSAccessibilityStartTextMarkerAttribute = @"AXStartTextMarker";
@@ skipping 2784 matching lines @@
   if (![self instanceActive])
     return [super hash];
   return browserAccessibility_->GetId();
 }
 
 - (BOOL)accessibilityShouldUseUniqueId {
   return YES;
 }
 
 @end