Introduce new API orientation() to expose aria-orientation correctly.

public/web/WebAXEnums.h

Index: public/web/WebAXEnums.h
diff --git a/public/web/WebAXEnums.h b/public/web/WebAXEnums.h
index a5e7439e3d0b2868a520bc4903a1b9fe9f9c2074..e09e5c1683dc64fd6a6636c99a537c4748b547c1 100644
--- a/public/web/WebAXEnums.h
+++ b/public/web/WebAXEnums.h
@@ -240,6 +240,14 @@ enum WebAXExpanded {
     WebAXExpandedExpanded
 };
 
+// These values must match blink::AccessibilityOrientation values.
+// Enforced in AssertMatchingEnums.cpp.
+enum WebAXOrientation {
+    WebAXOrientationUndefined = 0,
+    WebAXOrientationVertical,
+    WebAXOrientationHorizontal,
+};
+
 } // namespace blink
 
 #endif