Fix accessibility constant clash in OSX 10.12.

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 900686dcea03ba6b1791b4ad7ce9006b0a793139..b6d73a815fff640a1df856ecb587d1396f53aafc 100644
--- a/content/browser/accessibility/browser_accessibility_cocoa.mm
+++ b/content/browser/accessibility/browser_accessibility_cocoa.mm
@@ -53,7 +53,6 @@ NSString* const NSAccessibilityInvalidAttribute = @"AXInvalid";
 NSString* const NSAccessibilityIsMultiSelectableAttribute =
     @"AXIsMultiSelectable";
 NSString* const NSAccessibilityLoadingProgressAttribute = @"AXLoadingProgress";
-NSString* const NSAccessibilityRequiredAttribute = @"AXRequired";
 NSString* const
     NSAccessibilityUIElementCountForSearchPredicateParameterizedAttribute =
         @"AXUIElementCountForSearchPredicate";
@@ -504,6 +503,14 @@ bool InitializeAccessibilityTreeSearch(
 
 }  // namespace
 
+// The following private WebKit accessibility attribute became public in 10.12.
+#if !defined(MAC_OS_X_VERSION_10_12) || \
+    MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_X_VERSION_10_12
+extern "C" {
+NSString* const NSAccessibilityRequiredAttribute = @"AXRequired";
+}
+#endif  // MAC_OS_X_VERSION_10_12
+
 @implementation BrowserAccessibilityCocoa
 
 + (void)initialize {