Fix improper cast from PipeWire portal to RequestResponse

When ScreencastPortal::OnStartRequestResponseSignal receives either a
non-zero response code or is missing the response data, it would
directly cast this to a RequestResponse. However, this direct cast is an
error. Per the documentation, the response signal returns the following
values with their corresponding meanings:
0 - Success
1 - User Cancelled
2 - Error

The RequestResponse enum however, has "kUnknown" as 0, and thus
"kSuccess" as 1 (with all other values also shifted up by 1 value). This
means that when the portal was cancelled, we were still receiving
RequestResponse::kSuccess. This fixes the issue by removing the improper
cast and adding a translation function. This function is local for now
since no where else attempted to cast values to a RequestResponse; but
can be moved if the need arises.

Fixed: chromium:1351824
Change-Id: I4cd44d90055147c9592d590c7969dcfc3297a3d9
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/271240
Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Auto-Submit: Alexander Cooper <alcooper@chromium.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#37755}

modules/desktop_capture/linux/wayland/screencast_portal.cc

@@ -30,6 +30,21 @@ using xdg_portal::SetupSessionRequestHandlers;
 using xdg_portal::StartSessionRequest;
 using xdg_portal::TearDownSession;
 
+RequestResponse ToRequestResponseFromSignal(uint32_t signal_response) {
+  // See:
+  //  https://docs.flatpak.org/en/latest/portal-api-reference.html#gdbus-signal-org-freedesktop-portal-Request.Response
+  switch (signal_response) {
+    case 0:
+      return RequestResponse::kSuccess;
+    case 1:
+      return RequestResponse::kUserCancelled;
+    case 2:
+      return RequestResponse::kError;
+    default:
+      return RequestResponse::kUnknown;
+  }
+}
+
 }  // namespace
 
 ScreenCastPortal::ScreenCastPortal(
@@ -342,7 +357,7 @@ void ScreenCastPortal::OnStartRequestResponseSignal(GDBusConnection* connection,
                 response_data.receive());
   if (portal_response || !response_data) {
     RTC_LOG(LS_ERROR) << "Failed to start the screen cast session.";
-    that->OnPortalDone(static_cast<RequestResponse>(portal_response));
+    that->OnPortalDone(ToRequestResponseFromSignal(portal_response));
     return;
   }