Fix race condition (#20905)

shell/common/shell_unittests.cc

@@ -622,7 +622,22 @@ TEST_F(ShellTest, OnPlatformViewDestroyDisablesThreadMerger) {
 
   RunEngine(shell.get(), std::move(configuration));
 
-  PumpOneFrame(shell.get());
+  LayerTreeBuilder builder = [&](std::shared_ptr<ContainerLayer> root) {
+    SkPictureRecorder recorder;
+    SkCanvas* recording_canvas =
+        recorder.beginRecording(SkRect::MakeXYWH(0, 0, 80, 80));
+    recording_canvas->drawRect(SkRect::MakeXYWH(0, 0, 80, 80),
+                               SkPaint(SkColor4f::FromColor(SK_ColorRED)));
+    auto sk_picture = recorder.finishRecordingAsPicture();
+    fml::RefPtr<SkiaUnrefQueue> queue = fml::MakeRefCounted<SkiaUnrefQueue>(
+        this->GetCurrentTaskRunner(), fml::TimeDelta::FromSeconds(0));
+    auto picture_layer = std::make_shared<PictureLayer>(
+        SkPoint::Make(10, 10),
+        flutter::SkiaGPUObject<SkPicture>({sk_picture, queue}), false, false);
+    root->Add(picture_layer);
+  };
+
+  PumpOneFrame(shell.get(), 100, 100, builder);
 
   end_frame_latch.Wait();
   ASSERT_TRUE(raster_thread_merger->IsEnabled());