Consistently cache ElementData::length() before loops

Source/core/dom/Element.h

Index: Source/core/dom/Element.h
diff --git a/Source/core/dom/Element.h b/Source/core/dom/Element.h
index e4a49bc025def86a4edbcab79a95eda96eabf97a..52bb2ba0778c8914fe9d6a02dbc7fee2ba55dad1 100644
--- a/Source/core/dom/Element.h
+++ b/Source/core/dom/Element.h
@@ -154,7 +154,8 @@ public:
     const AtomicString& idForStyleResolution() const;
 
     // Internal methods that assume the existence of attribute storage, one should use hasAttributes()
-    // before calling them.
+    // before calling them. This is not a trivial getter and its return value should be cached for
+    // performance.
     size_t attributeCount() const;
     const Attribute* attributeItem(unsigned index) const;
     const Attribute* getAttributeItem(const QualifiedName&) const;