Avoid copying the CachedUAStyle when possible

Source/core/css/resolver/StyleResolverState.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  *
  * 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
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 76 matching lines @@
     void setApplyPropertyToVisitedLinkStyle(bool isApply) { m_applyPropertyToVisitedLinkStyle = isApply; }
     bool applyPropertyToRegularStyle() const { return m_applyPropertyToRegularStyle; }
     bool applyPropertyToVisitedLinkStyle() const { return m_applyPropertyToVisitedLinkStyle; }
 
     // Holds all attribute names found while applying "content" properties that contain an "attr()" value.
     Vector<AtomicString>& contentAttrValues() { return m_contentAttrValues; }
 
     void setLineHeightValue(CSSValue* value) { m_lineHeightValue = value; }
     CSSValue* lineHeightValue() { return m_lineHeightValue; }
 
-    void cacheUserAgentBorderAndBackground() { m_cachedUAStyle = CachedUAStyle(style()); }
+    void cacheUserAgentBorderAndBackground()

[ojan, 2014/03/17 18:59:13]

Nit: this is now kind of big for the header file. Move to the cpp?

+    {
+        // RenderTheme only needs the cached style if it has an appearance,
+        // and constructing it is expensive so we avoid it if possible.
+        if (!style()->hasAppearance())
+            return;
+        m_cachedUAStyle = CachedUAStyle(style());
+    }
     const CachedUAStyle& cachedUAStyle() const { return m_cachedUAStyle; }
 
     ElementStyleResources& elementStyleResources() { return m_elementStyleResources; }
     const CSSToStyleMap& styleMap() const { return m_styleMap; }
     CSSToStyleMap& styleMap() { return m_styleMap; }
 
     // FIXME: Once styleImage can be made to not take a StyleResolverState
     // this convenience function should be removed. As-is, without this, call
     // sites are extremely verbose.
     PassRefPtr<StyleImage> styleImage(CSSPropertyID propertyId, CSSValue* value)
@@ skipping 54 matching lines @@
     // a back-pointer to this object.
     CSSToStyleMap m_styleMap;
     Vector<AtomicString> m_contentAttrValues;
 
     StyleRule* m_currentRule;
 };
 
 } // namespace WebCore
 
 #endif // StyleResolverState_h