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 235635df1ce9bc4315fe01b0c53ea8c11b258a37..215bc3a5a264febdb54aac3fb26fc76fb8a7ce34 100644
--- a/public/web/WebAXEnums.h
+++ b/public/web/WebAXEnums.h
@@ -249,6 +249,14 @@ enum WebAXOptionalBool {
     WebAXOptionalBoolFalse
 };
 
+// These values must match blink::AccessibilityOrientation values.
+// Enforced in AssertMatchingEnums.cpp.
+enum WebAXOrientation {
+    WebAXOrientationUndefined = 0,
+    WebAXOrientationVertical,
+    WebAXOrientationHorizontal,
+};
+
 } // namespace blink
 
 #endif