[mus+ash] Implements hit-test client in Aura

ui/aura/mus/window_port_mus.h

Index: ui/aura/mus/window_port_mus.h
diff --git a/ui/aura/mus/window_port_mus.h b/ui/aura/mus/window_port_mus.h
index 57500d95b92c3dbc80d82a6549a6ab5500bbdac9..f3cacf4da7565b58295c966e1b1f419368f87240 100644
--- a/ui/aura/mus/window_port_mus.h
+++ b/ui/aura/mus/window_port_mus.h
@@ -58,6 +58,8 @@ class AURA_EXPORT WindowPortMus : public WindowPort, public WindowMus {
   Window* window() { return window_; }
   const Window* window() const { return window_; }
 
+  cc::FrameSinkId frame_sink_id() const { return frame_sink_id_; }
+
   ClientSurfaceEmbedder* client_surface_embedder() const {
     return client_surface_embedder_.get();
   }
@@ -98,6 +100,7 @@ class AURA_EXPORT WindowPortMus : public WindowPort, public WindowMus {
   friend class WindowTreeClient;
   friend class WindowTreeClientPrivate;
   friend class WindowTreeHostMus;
+  friend class HitTestDataProviderMusTest;
 
   using ServerChangeIdType = uint8_t;