aura-mus: Implement Deactivate().

ui/aura/mus/focus_synchronizer.cc

Index: ui/aura/mus/focus_synchronizer.cc
diff --git a/ui/aura/mus/focus_synchronizer.cc b/ui/aura/mus/focus_synchronizer.cc
index b211315fc9ca0c97d991b01993559d1fe0cd4d51..e775aa9ed1910e1d36ef3c296bbcb9174a8024bb 100644
--- a/ui/aura/mus/focus_synchronizer.cc
+++ b/ui/aura/mus/focus_synchronizer.cc
@@ -46,6 +46,10 @@ void FocusSynchronizer::SetFocusFromServer(WindowMus* window) {
   }
 }
 
+void FocusSynchronizer::ClearFocus() {
+  SetFocusedWindow(nullptr);
+}
+
 void FocusSynchronizer::OnFocusedWindowDestroyed() {
   focused_window_ = nullptr;
 }