Cache length in ElementData::getAttributeItemIndex() for performance

Source/core/dom/ElementData.h

Index: Source/core/dom/ElementData.h
diff --git a/Source/core/dom/ElementData.h b/Source/core/dom/ElementData.h
index b7c36f097a53720d0e80ec561c4a7e6ceb3c81ff..3e69d994125949b91cdda2e0a0f84eb8e4f8729a 100644
--- a/Source/core/dom/ElementData.h
+++ b/Source/core/dom/ElementData.h
@@ -185,7 +185,8 @@ inline const Attribute* ElementData::attributeBase() const
 inline size_t ElementData::getAttributeItemIndex(const QualifiedName& name, bool shouldIgnoreCase) const
 {
     const Attribute* begin = attributeBase();
-    for (unsigned i = 0; i < length(); ++i) {
+    unsigned len = length();
+    for (unsigned i = 0; i < len; ++i) {
         const Attribute& attribute = begin[i];
         if (attribute.name().matchesPossiblyIgnoringCase(name, shouldIgnoreCase))
             return i;