Bug 1376107 - Fix macOS AXRoleDescription for DPub ARIA AXLandmarkRegion. r=marcoz

DPub ARIA AXLandmarkRegion-maped roles should have an AXRoleDescription
value of "region"; not "group." This requires an explicit mapping, which
was not done during the initial implementation. No new test cases because
we currently have no means to test platform-accessibility-API mappngs via
mochitest.

accessible/mac/mozAccessible.mm

@@ -955,6 +955,7 @@ static const RoleDescrMap sRoleDescrMap[] = {
   { @"AXLandmarkContentInfo", NS_LITERAL_STRING("content") },
   { @"AXLandmarkMain", NS_LITERAL_STRING("main") },
   { @"AXLandmarkNavigation", NS_LITERAL_STRING("navigation") },
+  { @"AXLandmarkRegion", NS_LITERAL_STRING("region") },
   { @"AXLandmarkSearch", NS_LITERAL_STRING("search") },
   { @"AXSearchField", NS_LITERAL_STRING("searchTextField") },
   { @"AXTabPanel", NS_LITERAL_STRING("tabPanel") },