Make style building for font-weight less custom.

Source/platform/fonts/FontDescription.h

 /*
  * Copyright (C) 2000 Lars Knoll (knoll@kde.org)
  *           (C) 2000 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Nicholas Shanks <webkit@nickshanks.com>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
@@ skipping 95 matching lines @@
     const FontFamily& family() const { return m_familyList; }
     FontFamily& firstFamily() { return m_familyList; }
     float specifiedSize() const { return m_specifiedSize; }
     float computedSize() const { return m_computedSize; }
     FontStyle style() const { return static_cast<FontStyle>(m_style); }
     int computedPixelSize() const { return int(m_computedSize + 0.5f); }
     FontVariant variant() const { return static_cast<FontVariant>(m_variant); }
     bool isAbsoluteSize() const { return m_isAbsoluteSize; }
     FontWeight weight() const { return static_cast<FontWeight>(m_weight); }
     FontStretch stretch() const { return static_cast<FontStretch>(m_stretch); }
-    FontWeight lighterWeight() const;
-    FontWeight bolderWeight() const;
+    static FontWeight lighterWeight(FontWeight);
+    static FontWeight bolderWeight(FontWeight);
     GenericFamilyType genericFamily() const { return static_cast<GenericFamilyType>(m_genericFamily); }
 
     // only use fixed default size when there is only one font family, and that family is "monospace"
     FixedPitchFontType fixedPitchFontType() const
     {
         if (genericFamily() == MonospaceFamily && !family().next() && family().family() == FontFamilyNames::webkit_monospace)
             return FixedPitchFont;
         return NonFixedPitchFont;
     }
     Kerning kerning() const { return static_cast<Kerning>(m_kerning); }
@@ skipping 136 matching lines @@
         && m_script == other.m_script
         && m_syntheticBold == other.m_syntheticBold
         && m_syntheticItalic == other.m_syntheticItalic
         && m_featureSettings == other.m_featureSettings
         && m_subpixelTextPosition == other.m_subpixelTextPosition;
 }
 
 }
 
 #endif