Mus: Fixes rendering of transparent containers.

components/mus/ws/test_utils.h

Index: components/mus/ws/test_utils.h
diff --git a/components/mus/ws/test_utils.h b/components/mus/ws/test_utils.h
index 21cce2bfe80f51921f122437bb142ba0fe1bc4ba..3e1fab8da9d620a62cee2c4c4e02c638a4a925e7 100644
--- a/components/mus/ws/test_utils.h
+++ b/components/mus/ws/test_utils.h
@@ -226,6 +226,26 @@ class TestPlatformDisplayFactory : public PlatformDisplayFactory {
 
 // -----------------------------------------------------------------------------
 
+// A stub implementation of FrameGeneratorDelegate.
+class TestFrameGeneratorDelegate : public FrameGeneratorDelegate {
+ public:
+  explicit TestFrameGeneratorDelegate(std::unique_ptr<ServerWindow> root);
+  ~TestFrameGeneratorDelegate() override;
+
+  // FrameGeneratorDelegate:
+  ServerWindow* GetRootWindow() override;
+  void OnCompositorFrameDrawn() override {}
+  const ViewportMetrics& GetViewportMetrics() override;
+
+ private:
+  std::unique_ptr<ServerWindow> root_;
+  ViewportMetrics metrics_;
+
+  DISALLOW_COPY_AND_ASSIGN(TestFrameGeneratorDelegate);
+};
+
+// -----------------------------------------------------------------------------
+
 class TestWindowManager : public mojom::WindowManager {
  public:
   TestWindowManager()