Makes WindowTree::SetFocus(null) work

ui/views/mus/native_widget_mus.cc

Index: ui/views/mus/native_widget_mus.cc
diff --git a/ui/views/mus/native_widget_mus.cc b/ui/views/mus/native_widget_mus.cc
index c501c6e6b843dd4bb60e8ccbbba3b23f4d0526f3..751912f9a257691a5bacb0fc970116e47658db70 100644
--- a/ui/views/mus/native_widget_mus.cc
+++ b/ui/views/mus/native_widget_mus.cc
@@ -690,12 +690,14 @@ void NativeWidgetMus::Activate() {
 }
 
 void NativeWidgetMus::Deactivate() {
-  // NOTIMPLEMENTED();
+  if (IsActive())
+    window_->connection()->ClearFocus();
 }
 
 bool NativeWidgetMus::IsActive() const {
-  // NOTIMPLEMENTED();
-  return true;
+  mus::Window* focused =
+      window_ ? window_->connection()->GetFocusedWindow() : nullptr;
+  return focused && window_->Contains(focused);
 }
 
 void NativeWidgetMus::SetAlwaysOnTop(bool always_on_top) {
@@ -807,7 +809,12 @@ bool NativeWidgetMus::IsMouseEventsEnabled() const {
 }
 
 void NativeWidgetMus::ClearNativeFocus() {
-  // NOTIMPLEMENTED();
+  if (!IsActive())
+    return;
+  mus::Window* focused =
+      window_ ? window_->connection()->GetFocusedWindow() : nullptr;
+  if (focused && window_->Contains(focused) && focused != window_)
+    window_->SetFocus();
 }
 
 gfx::Rect NativeWidgetMus::GetWorkAreaBoundsInScreen() const {