MPRIS: skip the seek signal for infinite-duration position states

YouTube calls setPositionState({duration: Infinity}) every ~15 s. Each one
resets the MPRIS position and emits a seek signal, which makes gnome-shell run a
full JS GC that stalls mutter for ~37 ms and drops video frames.

--- a/components/system_media_controls/linux/system_media_controls_linux.cc
+++ b/components/system_media_controls/linux/system_media_controls_linux.cc
@@ -548,10 +548,16 @@
 
   if (!service_ready_ || !emit_signal || !position_.has_value()) {
     return;
   }
 
+  // An infinite duration has no seekable position. YouTube resets such a state
+  // every ~15 s, and each seek signal triggers a full JS GC in gnome-shell.
+  if (position_->duration().is_inf()) {
+    return;
+  }
+
   dbus::Signal seeked_signal(kMprisAPIPlayerInterfaceName,
                              kMprisAPISignalSeeked);
   dbus::MessageWriter writer(&seeked_signal);
   writer.AppendInt64(position);
   exported_object_->SendSignal(&seeked_signal);