aura-mus: Implement Deactivate().

ui/aura/mus/window_tree_client.cc

Index: ui/aura/mus/window_tree_client.cc
diff --git a/ui/aura/mus/window_tree_client.cc b/ui/aura/mus/window_tree_client.cc
index 6f6c35090eac6685e39216ac8f3fa86a5eb27309..65b68b1dddb19cb7eb4361d758e972dd63dffbe0 100644
--- a/ui/aura/mus/window_tree_client.cc
+++ b/ui/aura/mus/window_tree_client.cc
@@ -1580,6 +1580,10 @@ void WindowTreeClient::OnWindowTreeHostHitTestMaskWillChange(
                         out_rect);
 }
 
+void WindowTreeClient::OnWindowTreeHostClearFocus() {
+  focus_synchronizer_->ClearFocus();
+}
+
 std::unique_ptr<WindowPortMus> WindowTreeClient::CreateWindowPortForTopLevel(
     const std::map<std::string, std::vector<uint8_t>>* properties) {
   std::unique_ptr<WindowPortMus> window_port =