Add a FOUC painting test.

Source/web/tests/sim/SimCompositor.h

Index: Source/web/tests/sim/SimCompositor.h
diff --git a/Source/web/tests/sim/SimCompositor.h b/Source/web/tests/sim/SimCompositor.h
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index 0000000000000000000000000000000000000000..53975e3be79c44bfcc8f788946398fcb4dd57f42
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+++ b/Source/web/tests/sim/SimCompositor.h
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+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef SimCompositor_h
+#define SimCompositor_h
+
+namespace blink {
+
+class SimDisplayItemList;
+class SimLayerTreeView;
+class WebViewImpl;
+
+// Simulated very basic compositor that's capable of running the BeginMainFrame
+// processing steps on WebView: beginFrame, layout, paint.
+//
+// The painting capabilities are very limited in that only the main layer of
+// every CompositedDeprecatedPaintLayerMapping will be painted, squashed layers
+// are not supported and the entirety of every layer is always repainted even if
+// only part of the layer was invalid.
+//
+// Note: This also does not support compositor driven animations.
+class SimCompositor final {
+public:
+    explicit SimCompositor(SimLayerTreeView&);
+    ~SimCompositor();
+
+    void setWebViewImpl(WebViewImpl&);
+
+    // Execute the BeginMainFrame processing steps, an approximation of what
+    // cc::ThreadProxy::BeginMainFrame would do.
+    SimDisplayItemList beginFrame();
+
+private:
+    SimLayerTreeView* m_layerTreeView;
+    WebViewImpl* m_webViewImpl;
+    double m_lastFrameTimeMonotonic;
+};
+
+} // namespace blink
+
+#endif