Use a new Supplement constructor for Supplement<Document> (Part 1)

third_party/WebKit/Source/core/dom/DocumentParserTiming.cpp

 // Copyright 2015 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/DocumentParserTiming.h"
 
 #include "core/dom/Document.h"
 #include "core/loader/DocumentLoader.h"
 #include "platform/instrumentation/tracing/TraceEvent.h"
 
@@ skipping 47 matching lines @@
     bool scriptInsertedViaDocumentWrite) {
   if (m_parserDetached || m_parserStart == 0.0 || m_parserStop > 0.0)
     return;
   m_parserBlockedOnScriptExecutionDuration += duration;
   if (scriptInsertedViaDocumentWrite)
     m_parserBlockedOnScriptExecutionFromDocumentWriteDuration += duration;
   notifyDocumentParserTimingChanged();
 }
 
 DEFINE_TRACE(DocumentParserTiming) {
-  visitor->trace(m_document);
   Supplement<Document>::trace(visitor);
 }
 
 DocumentParserTiming::DocumentParserTiming(Document& document)
-    : m_document(document) {}
+    : Supplement<Document>(document) {}
 
 void DocumentParserTiming::notifyDocumentParserTimingChanged() {
-  if (m_document->loader())
-    m_document->loader()->didChangePerformanceTiming();
+  if (host()->loader())
+    host()->loader()->didChangePerformanceTiming();
 }
 
 }  // namespace blink