Mus: Fixes rendering of transparent containers.

components/mus/ws/frame_generator.h

Index: components/mus/ws/frame_generator.h
diff --git a/components/mus/ws/frame_generator.h b/components/mus/ws/frame_generator.h
index 9c31f3552043a00d2b892c5fc83762d7e0be6cf4..93aff938149a338ed6f673ea8382de95a0b80525 100644
--- a/components/mus/ws/frame_generator.h
+++ b/components/mus/ws/frame_generator.h
@@ -27,6 +27,10 @@ class SurfacesState;
 
 namespace ws {
 
+namespace test {
+class FrameGeneratorTest;
+}
+
 class FrameGeneratorDelegate;
 class ServerWindow;
 
@@ -48,6 +52,8 @@ class FrameGenerator {
   bool is_frame_pending() { return frame_pending_; }
 
  private:
+  friend class mus::ws::test::FrameGeneratorTest;
+
   void WantToDraw();
 
   // This method initiates a top level redraw of the display.