Bug 1682121 - Add a ToJSValue overload for nsACString. r=hsivonen

I wrote this for bug 1682003, but I split it off it because I don't
really _need_ it (I need it on one place where I know I have valid
utf-8).

I think this is a sensible thing to do though. This throws on invalid
utf-8 via:

  https://searchfox.org/mozilla-central/rev/0bcf81557b89e7757c44e25bb4bc7f4cb8619dc9/js/src/vm/CharacterEncoding.cpp#432

This would allow doing stuff like resolving promises using nsCStrings
slightly more conveniently / efficiently than using NS_ConvertUTF*.

Differential Revision: https://phabricator.services.mozilla.com/D99589

dom/bindings/ToJSValue.cpp

@@ -36,6 +36,11 @@ bool ToJSValue(JSContext* aCx, const nsAString& aArgument,
   return true;
 }
 
+bool ToJSValue(JSContext* aCx, const nsACString& aArgument,
+               JS::MutableHandle<JS::Value> aValue) {
+  return UTF8StringToJsval(aCx, aArgument, aValue);
+}
+
 bool ToJSValue(JSContext* aCx, nsresult aArgument,
                JS::MutableHandle<JS::Value> aValue) {
   RefPtr<Exception> exception = CreateException(aArgument);

dom/bindings/ToJSValue.h

@@ -46,6 +46,10 @@ class TypedArrayCreator;
 MOZ_MUST_USE bool ToJSValue(JSContext* aCx, const nsAString& aArgument,
                             JS::MutableHandle<JS::Value> aValue);
 
+// Treats the input as UTF-8, and throws otherwise.
+MOZ_MUST_USE bool ToJSValue(JSContext* aCx, const nsACString& aArgument,
+                            JS::MutableHandle<JS::Value> aValue);
+
 // Accept booleans.  But be careful here: if we just have a function that takes
 // a boolean argument, then any pointer that doesn't match one of our other
 // signatures/templates will get treated as a boolean, which is clearly not