[fuchsia] Change vsync trace name back to "vsync callback" on Fuchsia (#17118)

Fuchsia benchmarks depend the name being precisely "vsync callback".
Since this is causing immediate build issues, change it back to "vsync
callback" on Fuchsia.  Stronger testing for this in the long run is
currently under discussion.

b/150902962

shell/common/vsync_waiter.cc

@@ -11,6 +11,24 @@ namespace flutter {
 
 static constexpr const char* kVsyncFlowName = "VsyncFlow";
 
+#if defined(OS_FUCHSIA)
+//  ________  _________  ________  ________
+// |\   ____\|\___   ___\\   __  \|\   __  \
+// \ \  \___|\|___ \  \_\ \  \|\  \ \  \|\  \
+//  \ \_____  \   \ \  \ \ \  \\\  \ \   ____\
+//   \|____|\  \   \ \  \ \ \  \\\  \ \  \___|
+//     ____\_\  \   \ \__\ \ \_______\ \__\
+//    |\_________\   \|__|  \|_______|\|__|
+//    \|_________|
+//
+// Fuchsia benchmarks depend on this trace event's name.  Please do not change
+// it without checking that the changes are compatible with Fuchsia benchmarks
+// first!
+static constexpr const char* kVsyncTraceName = "vsync callback";
+#else
+static constexpr const char* kVsyncTraceName = "VsyncProcessCallback";
+#endif
+
 VsyncWaiter::VsyncWaiter(TaskRunners task_runners)
     : task_runners_(std::move(task_runners)) {}
 
@@ -102,7 +120,7 @@ void VsyncWaiter::FireCallback(fml::TimePoint frame_start_time,
 
     task_runners_.GetUITaskRunner()->PostTaskForTime(
         [callback, flow_identifier, frame_start_time, frame_target_time]() {
-          FML_TRACE_EVENT("flutter", "VsyncProcessCallback", "StartTime",
+          FML_TRACE_EVENT("flutter", kVsyncTraceName, "StartTime",
                           frame_start_time, "TargetTime", frame_target_time);
           fml::tracing::TraceEventAsyncComplete(
               "flutter", "VsyncSchedulingOverhead", fml::TimePoint::Now(),