Bug 1888595 - Auto directionality of non-value-associated form elements should always be LTR. r=smaug

https://html.spec.whatwg.org/#auto-directionality always returns null,
effectively, so should default to always LTR. Inheriting of the parent
directionality wouldn't be handled correctly because we stop the
inheritance on dir=auto boundaries.

Differential Revision: https://phabricator.services.mozilla.com/D206236

dom/html/HTMLInputElement.cpp

@@ -5921,7 +5921,7 @@ nsresult HTMLInputElement::SetDefaultValueAsValue() {
 void HTMLInputElement::SetAutoDirectionality(bool aNotify,
                                              const nsAString* aKnownValue) {
   if (!IsAutoDirectionalityAssociated()) {
-    return SetDirectionality(GetParentDirectionality(this), aNotify);
+    return SetDirectionality(Directionality::Ltr, aNotify);
   }
   nsAutoString value;
   if (!aKnownValue) {