Record the reference time when Performance is constructed

Source/core/page/Performance.cpp

Index: Source/core/page/Performance.cpp
diff --git a/Source/core/page/Performance.cpp b/Source/core/page/Performance.cpp
index 0d9b274dfcd630d9e6c74e934b44f3328be20b94..c77484c46f696cdd180a1cb613d8e9f0bfe55452 100644
--- a/Source/core/page/Performance.cpp
+++ b/Source/core/page/Performance.cpp
@@ -54,7 +54,9 @@ 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);
 }
 
@@ -286,7 +288,7 @@ void Performance::clearMeasures(const String& measureName)
 
 double Performance::now() const
 {
-    return 1000.0 * m_frame->document()->loader()->timing()->monotonicTimeToZeroBasedDocumentTime(monotonicallyIncreasingTime());
+    return 1000.0 * (monotonicallyIncreasingTime() - m_referenceTime);
 }
 
 } // namespace WebCore