aura-mus: Implement Deactivate().

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 eb300a592e1cb0f77d949c0c921cadd7a2ee2034..ca5d986aad42bab1b419329eb18fe411a93aa131 100644
--- a/ui/aura/mus/window_tree_host_mus.cc
+++ b/ui/aura/mus/window_tree_host_mus.cc
@@ -122,6 +122,10 @@ void WindowTreeHostMus::SetHitTestMask(const base::Optional<gfx::Rect>& rect) {
   delegate_->OnWindowTreeHostHitTestMaskWillChange(this, rect);
 }
 
+void WindowTreeHostMus::DeactivateWindow() {
+  delegate_->OnWindowTreeHostDeactivateWindow(this);
+}
+
 display::Display WindowTreeHostMus::GetDisplay() const {
   display::Display display;
   display::Screen::GetScreen()->GetDisplayWithDisplayId(display_id_, &display);