Add Qt nsNativeBasicThemeQt class

Adds a Qt version of the nsNativeBasicThemeQt class, which is now needed
due to upstream changes.

This reflects similar changes applied upstream for other toolkits (e.g.
Gtk, Android).

widget/qt/moz.build

@@ -9,6 +9,7 @@ SOURCES += [
     'nsAppShell.cpp',
     'nsBidiKeyboard.cpp',
     'nsLookAndFeel.cpp',
+    'nsNativeBasicThemeQt.cpp',
     'nsNativeThemeQt.cpp',
     'nsScreenManagerQt.cpp',
     'nsScreenQt.cpp',

widget/qt/nsNativeBasicThemeQt.cpp

@@ -0,0 +1,17 @@
+/* -*- Mode: C++; tab-width: 40; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsNativeBasicThemeQt.h"
+
+#include "mozilla/ClearOnShutdown.h"
+
+already_AddRefed<nsITheme> do_GetBasicNativeThemeDoNotUseDirectly() {
+  static mozilla::StaticRefPtr<nsITheme> gInstance;
+  if (MOZ_UNLIKELY(!gInstance)) {
+    gInstance = new nsNativeBasicThemeQt();
+    ClearOnShutdown(&gInstance);
+  }
+  return do_AddRef(gInstance);
+}

widget/qt/nsNativeBasicThemeQt.h

@@ -0,0 +1,20 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsNativeBasicThemeQt_h
+#define nsNativeBasicThemeQt_h
+
+#include "nsNativeBasicTheme.h"
+
+class nsNativeBasicThemeQt : public nsNativeBasicTheme {
+ public:
+  nsNativeBasicThemeQt() = default;
+
+ protected:
+  virtual ~nsNativeBasicThemeQt() = default;
+};
+
+#endif