Bug 1672987 Use PipeWire when Wayland display is actually used, r=dminor

Right now PipeWire is enabled when Wayland session is used regardless of an active Gtk backend (X11/Wayland).
Let's use PipeWire only when Wayland Gtk backend is used and disable it for X11 one to avoid possible regressions.

Differential Revision: https://phabricator.services.mozilla.com/D94588

third_party/libwebrtc/webrtc/modules/desktop_capture/desktop_capturer.cc

@@ -13,6 +13,11 @@
 #include "modules/desktop_capture/desktop_capture_options.h"
 #include "modules/desktop_capture/desktop_capturer_differ_wrapper.h"
 
+#if defined(WEBRTC_USE_PIPEWIRE) || defined(USE_X11)
+#include <gtk/gtk.h>
+#include <gtk/gtkx.h>
+#endif
+
 namespace webrtc {
 
 DesktopCapturer::~DesktopCapturer() = default;
@@ -72,7 +77,19 @@ std::unique_ptr<DesktopCapturer> DesktopCapturer::CreateTabCapturer(
 }
 
 #if defined(WEBRTC_USE_PIPEWIRE) || defined(USE_X11)
-bool DesktopCapturer::IsRunningUnderWayland() {
+// Return true if Firefox is actually running with Wayland backend.
+static bool IsWaylandDisplayUsed() {
+  const auto display = gdk_display_get_default();
+  if (display == nullptr) {
+    // We're running in headless mode.
+    return false;
+  }
+  return !GDK_IS_X11_DISPLAY(display);
+}
+
+// Return true if Firefox is actually running on Wayland enabled session.
+// It means some screensharing capabilities may be limited.
+static bool IsWaylandSessionUsed() {
   const char* xdg_session_type = getenv("XDG_SESSION_TYPE");
   if (!xdg_session_type || strncmp(xdg_session_type, "wayland", 7) != 0)
     return false;
@@ -82,6 +99,10 @@ bool DesktopCapturer::IsRunningUnderWayland() {
 
   return true;
 }
+
+bool DesktopCapturer::IsRunningUnderWayland() {
+  return IsWaylandSessionUsed() ? IsWaylandDisplayUsed() : false;
+}
 #endif  // defined(WEBRTC_USE_PIPEWIRE) || defined(USE_X11)
 
 }  // namespace webrtc