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

third_party/WebKit/Source/core/html/parser/BackgroundHTMLInputStream.cpp

 /*
  * Copyright (C) 2013 Google, Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 97 matching lines @@
   m_segments.clear();
   m_checkpoints.clear();
   m_firstValidCheckpointIndex = 0;
   m_firstValidSegmentIndex = 0;
 
   updateTotalCheckpointTokenCount();
 }
 
 void BackgroundHTMLInputStream::updateTotalCheckpointTokenCount() {
   m_totalCheckpointTokenCount = 0;
-  size_t lastCheckpointIndex = m_checkpoints.size();
-  for (size_t i = 0; i < lastCheckpointIndex; ++i)
+  for (const auto& checkpoint : m_checkpoints) {
     m_totalCheckpointTokenCount +=
-        m_checkpoints[i].tokensExtractedSincePreviousCheckpoint;
+        checkpoint.tokensExtractedSincePreviousCheckpoint;
+  }
 }
 
 }  // namespace blink