[Impeller] Make DoMakeRasterSnapshot output timeline event. (#41197)

In Skia, there is such a timeline event in `SnapshotControllerSkia::DoMakeRasterSnapshot`. Therefore, since Impeller wants to mimic Skia and this event does take a long time sometimes, it seems reasonable to add this.

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: fzyzcjy

shell/common/snapshot_controller_impeller.cc

@@ -33,6 +33,7 @@ sk_sp<DlImage> SnapshotControllerImpeller::MakeRasterSnapshot(
 sk_sp<DlImage> SnapshotControllerImpeller::DoMakeRasterSnapshot(
     const sk_sp<DisplayList>& display_list,
     SkISize size) {
+  TRACE_EVENT0("flutter", __FUNCTION__);
   impeller::DisplayListDispatcher dispatcher;
   display_list->Dispatch(dispatcher);
   impeller::Picture picture = dispatcher.EndRecordingAsPicture();