Avoid WTF::Vector::at() and operator[] in core/xml.

third_party/WebKit/Source/core/xml/XPathNodeSet.h

Index: third_party/WebKit/Source/core/xml/XPathNodeSet.h
diff --git a/third_party/WebKit/Source/core/xml/XPathNodeSet.h b/third_party/WebKit/Source/core/xml/XPathNodeSet.h
index 00549aeadca676850200d339fcb821ef9724b264..6ba8ee1ac0e1f97aba9ce0ebd0e16d6c8a4b9d60 100644
--- a/third_party/WebKit/Source/core/xml/XPathNodeSet.h
+++ b/third_party/WebKit/Source/core/xml/XPathNodeSet.h
@@ -43,6 +43,12 @@ class NodeSet final : public GarbageCollected<NodeSet> {
   size_t size() const { return m_nodes.size(); }
   bool isEmpty() const { return !m_nodes.size(); }
   Node* operator[](unsigned i) const { return m_nodes.at(i).get(); }
+  HeapVector<Member<Node>>::iterator begin() { return m_nodes.begin(); }
+  HeapVector<Member<Node>>::iterator end() { return m_nodes.end(); }
+  HeapVector<Member<Node>>::const_iterator begin() const {
+    return m_nodes.begin();
+  }
+  HeapVector<Member<Node>>::const_iterator end() const { return m_nodes.end(); }
   void reserveCapacity(size_t newCapacity) {
     m_nodes.reserveCapacity(newCapacity);
   }