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

third_party/WebKit/Source/core/dom/custom/V0CustomElementMicrotaskQueueBase.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/dom/custom/V0CustomElementMicrotaskQueueBase.h"
 
 #include "core/dom/custom/V0CustomElementProcessingStack.h"
 
 namespace blink {
 
 void V0CustomElementMicrotaskQueueBase::dispatch() {
   RELEASE_ASSERT(!m_inDispatch);
   m_inDispatch = true;
   doDispatch();
   m_inDispatch = false;
 }
 
 DEFINE_TRACE(V0CustomElementMicrotaskQueueBase) {
   visitor->trace(m_queue);
 }
 
 #if !defined(NDEBUG)
 void V0CustomElementMicrotaskQueueBase::show(unsigned indent) {
-  for (unsigned q = 0; q < m_queue.size(); ++q) {
-    if (m_queue[q])
-      m_queue[q]->show(indent);
+  for (const auto& step : m_queue) {
+    if (step)
+      step->show(indent);
     else
       fprintf(stderr, "%*snull\n", indent, "");
   }
 }
 #endif
 
 }  // namespace blink