Fix virtual/override/final usage in Source/core/layout/.

Source/core/layout/LayoutThemeLinux.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef LayoutThemeLinux_h
 #define LayoutThemeLinux_h
 
 #include "core/layout/LayoutThemeDefault.h"
 
 namespace blink {
 
 class LayoutThemeLinux final : public LayoutThemeDefault {
 public:
     static PassRefPtr<LayoutTheme> create();
-    virtual String extraDefaultStyleSheet() override;
+    String extraDefaultStyleSheet() override;
 };
 
 } // namespace blink
 
 #endif // LayoutThemeLinux_h