aura-mus: Implement Deactivate().

ui/views/mus/desktop_window_tree_host_mus.cc

Index: ui/views/mus/desktop_window_tree_host_mus.cc
diff --git a/ui/views/mus/desktop_window_tree_host_mus.cc b/ui/views/mus/desktop_window_tree_host_mus.cc
index 8d441d911bc0579c9e80727edf4352220d9b73c0..86a66b4699b88a7f2fd469de9dafc9c35ba0881f 100644
--- a/ui/views/mus/desktop_window_tree_host_mus.cc
+++ b/ui/views/mus/desktop_window_tree_host_mus.cc
@@ -453,9 +453,8 @@ void DesktopWindowTreeHostMus::Activate() {
 }
 
 void DesktopWindowTreeHostMus::Deactivate() {
-  // TODO: Deactivate() means focus next window, that needs to go to mus.
-  // http://crbug.com/663618.
-  NOTIMPLEMENTED();
+  if (is_active_)
+    ClearFocus();
 }
 
 bool DesktopWindowTreeHostMus::IsActive() const {