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..af6e9d2aa4fd2f213cf1d0740cc3cebf8161825f 100644
--- a/content/browser/accessibility/browser_accessibility_cocoa.mm
+++ b/content/browser/accessibility/browser_accessibility_cocoa.mm
@@ -53,7 +53,10 @@ 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";