Add a FOUC painting test.

Source/web/tests/sim/SimLayerTreeView.h

Index: Source/web/tests/sim/SimLayerTreeView.h
diff --git a/Source/web/tests/sim/SimLayerTreeView.h b/Source/web/tests/sim/SimLayerTreeView.h
index 655a040f179d93a838a12e9e0d9da2d10dc937bd..fddc0e3b23c30b5101f1639b8e1222b9b0ded804 100644
--- a/Source/web/tests/sim/SimLayerTreeView.h
+++ b/Source/web/tests/sim/SimLayerTreeView.h
@@ -10,12 +10,13 @@
 
 namespace blink {
 
-class SimLayerTreeView : public WebLayerTreeView {
+class SimLayerTreeView final : public WebLayerTreeView {
 public:
     SimLayerTreeView();
 
     void setNeedsAnimate() override { m_needsAnimate = true; }
     bool needsAnimate() const { return m_needsAnimate; }
+    void resetNeedsAnimate() { m_needsAnimate = false; }
 
     void setDeferCommits(bool deferCommits) override { m_deferCommits = deferCommits; }
     bool deferCommits() const { return m_deferCommits; }