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

ui/aura/mus/window_tree_host_mus.cc

Index: ui/aura/mus/window_tree_host_mus.cc
diff --git a/ui/aura/mus/window_tree_host_mus.cc b/ui/aura/mus/window_tree_host_mus.cc
index 07c720c3da249a862c103d5bc83b1bc76239b262..6e15724504c3b9a78d7f25bad91b1e22199c74a7 100644
--- a/ui/aura/mus/window_tree_host_mus.cc
+++ b/ui/aura/mus/window_tree_host_mus.cc
@@ -146,6 +146,10 @@ void WindowTreeHostMus::SetHitTestMask(const base::Optional<gfx::Rect>& rect) {
   delegate_->OnWindowTreeHostHitTestMaskWillChange(this, rect);
 }
 
+void WindowTreeHostMus::SetOpacity(float value) {
+  delegate_->OnWindowTreeHostSetOpacity(this, value);
+}
+
 void WindowTreeHostMus::DeactivateWindow() {
   delegate_->OnWindowTreeHostDeactivateWindow(this);
 }