Remove the "VSYNC" trace event on Fuchsia (#5907)

Don't emit a "VSYNC" event when running on Fuchsia, as traces on Fuchsia
are typically recorded across the whole system, causing the events to
collide with each other.

shell/common/vsync_waiter.cc

@@ -37,10 +37,17 @@ void VsyncWaiter::FireCallback(fml::TimePoint frame_start_time,
 
   task_runners_.GetUITaskRunner()->PostTask(
       [callback, frame_start_time, frame_target_time]() {
+#if defined(OS_FUCHSIA)
+        // In general, traces on Fuchsia are recorded across the whole system.
+        // Because of this, emitting a "VSYNC" event per flutter process is
+        // undesirable, as the events will collide with each other.  We
+        // instead let another area of the system emit them.
+        TRACE_EVENT0("flutter", "vsync callback");
+#else
         // Note: The tag name must be "VSYNC" (it is special) so that the
-        // "Highlight
-        // Vsync" checkbox in the timeline can be enabled.
+        // "Highlight Vsync" checkbox in the timeline can be enabled.
         TRACE_EVENT0("flutter", "VSYNC");
+#endif
         callback(frame_start_time, frame_target_time);
       });
 }