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..2fa57a2da122d957f586ad186a2fa7dec83d669c 100644
--- a/Source/core/page/Performance.cpp
+++ b/Source/core/page/Performance.cpp
@@ -55,6 +55,11 @@ Performance::Performance(Frame* frame)
     , m_resourceTimingBufferSize(defaultResourceTimingBufferSize)
     , m_userTiming(0)
 {
+    ASSERT(m_frame);
+    ASSERT(m_frame->document());
+    ASSERT(m_frame->document()->loader());
+    ASSERT(m_frame->document()->loader()->timing()->referenceMonotonicTime());
+    m_referenceTime = m_frame->document()->loader()->timing()->referenceMonotonicTime();

[tonyg, 2013/08/23 00:01:06]

I don't think the asserts are going to teach you anything here and I suspect the
move will fix the crasher.

What do you think about moving this to the initializer list and removing the
asserts?

     ScriptWrappable::init(this);
 }
 
@@ -286,7 +291,7 @@ 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