Record the reference time when Performance is constructed

Source/core/page/Performance.cpp

 /*
  * Copyright (C) 2010 Google Inc. All rights reserved.
  * Copyright (C) 2012 Intel 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 36 matching lines @@
 #include "core/page/Frame.h"
 
 namespace WebCore {
 
 static const size_t defaultResourceTimingBufferSize = 150;
 
 Performance::Performance(Frame* frame)
     : DOMWindowProperty(frame)
     , m_resourceTimingBufferSize(defaultResourceTimingBufferSize)
     , m_userTiming(0)
+    , m_referenceTime(frame->document()->loader()->timing()->referenceMonotonicTime())
 {
+    ASSERT(m_referenceTime);
     ScriptWrappable::init(this);
 }
 
 Performance::~Performance()
 {
 }
 
 const AtomicString& Performance::interfaceName() const
 {
     return eventNames().interfaceForPerformance;
@@ skipping 211 matching lines @@
 
 void Performance::clearMeasures(const String& measureName)
 {
     if (!m_userTiming)
         m_userTiming = UserTiming::create(this);
     m_userTiming->clearMeasures(measureName);
 }
 
 double Performance::now() const
 {
-    return 1000.0 * m_frame->document()->loader()->timing()->monotonicTimeToZeroBasedDocumentTime(monotonicallyIncreasingTime());
+    return 1000.0 * (monotonicallyIncreasingTime() - m_referenceTime);
 }
 
 } // namespace WebCore