[Presentation API] reject request.start() with OperationError instead of UnknownError if previous s…

content/browser/presentation/presentation_service_impl.cc

Index: content/browser/presentation/presentation_service_impl.cc
diff --git a/content/browser/presentation/presentation_service_impl.cc b/content/browser/presentation/presentation_service_impl.cc
index 9d870e3adf4c717129a36ad5d51f12538394c494..3da5b3bed61b6b1849513c630222e556609b8204 100644
--- a/content/browser/presentation/presentation_service_impl.cc
+++ b/content/browser/presentation/presentation_service_impl.cc
@@ -109,7 +109,9 @@ void InvokeNewSessionCallbackWithError(
     const PresentationServiceImpl::NewSessionCallback& callback) {
   callback.Run(blink::mojom::PresentationSessionInfoPtr(),
                blink::mojom::PresentationError::From(PresentationError(
-                   PRESENTATION_ERROR_UNKNOWN, "Internal error")));
+                   PRESENTATION_ERROR_PREVIOUS_START_IN_PROGRESS,
+                   "There is already an unsettled Promise from a previous call "
+                   "to start.")));
 }
 
 }  // namespace