[tracing] SceneDisplayLag is a synchronous event (#17878)

This change converts it from an events that spans a time interval
to an event that occurs at an instant.

We also emit this trace event when there is no lag as opposed to
only when there was a lag to make it monotonous.
Co-authored-by: NKaushik Iska <kaushikiska@google.com>

shell/common/rasterizer.cc

@@ -286,6 +286,7 @@ RasterStatus Rasterizer::DoDraw(
   timing.Set(FrameTiming::kRasterFinish, raster_finish_time);
   delegate_.OnFrameRasterized(timing);
 
+  std::string vsync_transitions_missed = "0";
   if (raster_finish_time > frame_target_time) {
     fml::TimePoint latest_frame_target_time =
         delegate_.GetLatestFrameTargetTime();
@@ -297,21 +298,16 @@ RasterStatus Rasterizer::DoDraw(
     }
     const auto frame_lag =
         (latest_frame_target_time - frame_target_time).ToMillisecondsF();
-    const int vsync_transitions_missed = round(frame_lag / frame_budget_millis);
-    fml::tracing::TraceEventAsyncComplete(
-        "flutter",                    // category
-        "SceneDisplayLag",            // name
-        raster_finish_time,           // begin_time
-        latest_frame_target_time,     // end_time
-        "frame_target_time",          // arg_key_1
-        frame_target_time,            // arg_val_1
-        "current_frame_target_time",  // arg_key_2
-        latest_frame_target_time,     // arg_val_2
-        "vsync_transitions_missed",   // arg_key_3
-        vsync_transitions_missed      // arg_val_3
-    );
+    vsync_transitions_missed =
+        std::to_string(round(frame_lag / frame_budget_millis));
   }
 
+  TRACE_EVENT1("flutter",                        // cateogry
+               "SceneDisplayLag",                // name
+               "vsync_transitions_missed",       // arg1_key
+               vsync_transitions_missed.c_str()  // arg1_val
+  );
+
   // Pipeline pressure is applied from a couple of places:
   // rasterizer: When there are more items as of the time of Consume.
   // animator (via shell): Frame gets produces every vsync.