Bug 1376111 - Fix AXRoleDescription for ARIA heading role. r=MarcoZ

The AXRoleDescription should be "heading" not "AXHeading." Add a new
localized string and use it for roles::HEADING.

accessible/mac/mozAccessible.mm

@@ -977,6 +977,9 @@ struct RoleDescrComparator
   if (mRole == roles::DOCUMENT)
     return utils::LocalizedString(NS_LITERAL_STRING("htmlContent"));
 
+  if (mRole == roles::HEADING)
+    return utils::LocalizedString(NS_LITERAL_STRING("heading"));
+
   NSString* subrole = [self subrole];
 
   if (subrole) {

dom/locales/en-US/chrome/accessibility/mac/accessible.properties

@@ -40,6 +40,9 @@ alert       =      alert
 alertDialog =      alert dialog
 article     =      article
 document    =      document
+# The (spoken) role description for the WAI-ARIA heading role
+# https://w3c.github.io/aria/core-aam/core-aam.html#role-map-heading
+heading     =      heading
 log         =      log
 marquee     =      marquee
 math        =      math