aura-mus: Implement DesktopWindowTreeHostMus::SetOpacity().

ui/aura/mus/window_tree_host_mus_delegate.h

Index: ui/aura/mus/window_tree_host_mus_delegate.h
diff --git a/ui/aura/mus/window_tree_host_mus_delegate.h b/ui/aura/mus/window_tree_host_mus_delegate.h
index 08d8f58f387fd75a2acdadade23dbcd360150a94..b93c65980951e2b6b3572456c1db2067ab81d747 100644
--- a/ui/aura/mus/window_tree_host_mus_delegate.h
+++ b/ui/aura/mus/window_tree_host_mus_delegate.h
@@ -42,6 +42,10 @@ class AURA_EXPORT WindowTreeHostMusDelegate {
       WindowTreeHostMus* window_tree_host,
       const base::Optional<gfx::Rect>& mask_rect) = 0;
 
+  // Called when the opacity is changed client side.
+  virtual void OnWindowTreeHostSetOpacity(WindowTreeHostMus* window_tree_host,
+                                          float opacity) = 0;
+
   // Called to clear the focus of the current window.
   virtual void OnWindowTreeHostDeactivateWindow(
       WindowTreeHostMus* window_tree_host) = 0;