aura-mus: Implement Deactivate().

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 dd0be599f9cdd4f057ed0a81dfd2d72aed31dc05..b240d843fb8f70ae6cfd9f5bb38ad0a2131ba4ef 100644
--- a/ui/aura/mus/window_tree_host_mus_delegate.h
+++ b/ui/aura/mus/window_tree_host_mus_delegate.h
@@ -42,6 +42,9 @@ class AURA_EXPORT WindowTreeHostMusDelegate {
       WindowTreeHostMus* window_tree_host,
       const base::Optional<gfx::Rect>& mask_rect) = 0;
 
+  // Called to clear the focus of the current window.
+  virtual void OnWindowTreeHostClearFocus() = 0;
+
   // Called when a WindowTreeHostMus is created without a WindowPort.
   // TODO: this should take an unordered_map, see http://crbug.com/670515.
   virtual std::unique_ptr<WindowPortMus> CreateWindowPortForTopLevel(