aura-mus: Implement Deactivate().

ui/aura/mus/window_tree_host_mus.h

Index: ui/aura/mus/window_tree_host_mus.h
diff --git a/ui/aura/mus/window_tree_host_mus.h b/ui/aura/mus/window_tree_host_mus.h
index e29f7b0423bde2b537341b2abef873fcf3a5282f..389173eb67f623a3d21cbb6379744629dc70f314 100644
--- a/ui/aura/mus/window_tree_host_mus.h
+++ b/ui/aura/mus/window_tree_host_mus.h
@@ -67,6 +67,9 @@ class AURA_EXPORT WindowTreeHostMus : public aura::WindowTreeHostPlatform {
   // clear.
   void SetHitTestMask(const base::Optional<gfx::Rect>& rect);
 
+  // Sends a focus change to the mus server with a null window.
+  void ClearFocus();
+
   // Intended only for WindowTreeClient to call.
   void set_display_id(int64_t id) { display_id_ = id; }
   int64_t display_id() const { return display_id_; }