Mus: Fixes rendering of transparent containers.

components/mus/ws/test_utils.cc

Index: components/mus/ws/test_utils.cc
diff --git a/components/mus/ws/test_utils.cc b/components/mus/ws/test_utils.cc
index a371a0bb4708f24d86a1cfebdcbfad4651a3f8ae..1015495953fe884188b930d834f1488c1d814e60 100644
--- a/components/mus/ws/test_utils.cc
+++ b/components/mus/ws/test_utils.cc
@@ -107,6 +107,18 @@ PlatformDisplay* TestPlatformDisplayFactory::CreatePlatformDisplay() {
   return new TestPlatformDisplay(cursor_id_storage_);
 }
 
+// TestPlatformDisplayDelegate -------------------------------------------------
+
+TestPlatformDisplayDelegate::TestPlatformDisplayDelegate(
+    std::unique_ptr<ServerWindow> root)
+    : root_(std::move(root)) {}
+
+TestPlatformDisplayDelegate::~TestPlatformDisplayDelegate() {}
+
+ServerWindow* TestPlatformDisplayDelegate::GetRootWindow() {
+  return root_.get();
+}
+
 // WindowTreeTestApi  ---------------------------------------------------------
 
 WindowTreeTestApi::WindowTreeTestApi(WindowTree* tree) : tree_(tree) {}