Avoid copying the CachedUAStyle when possible

Source/core/css/resolver/StyleResolverState.h

Index: Source/core/css/resolver/StyleResolverState.h
diff --git a/Source/core/css/resolver/StyleResolverState.h b/Source/core/css/resolver/StyleResolverState.h
index 4e01e8c4fa48bc016e62a86868ad8b56e1a2db45..a23f9da7bd9f7b7d7bc7d54b5d91a7d452de4c01 100644
--- a/Source/core/css/resolver/StyleResolverState.h
+++ b/Source/core/css/resolver/StyleResolverState.h
@@ -94,7 +94,14 @@ public:
     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; }