Bug 1403926 - Make the PerformanceNavigationTiming timestamps be relative to startTime r=baku

MozReview-Commit-ID: 7lvSsde9dzh

--HG--
extra : rebase_source : bc4b7fa0526a3fe749c17ce10f70baa8a33f8bb1

dom/performance/PerformanceMainThread.cpp

@@ -314,7 +314,9 @@ PerformanceMainThread::EnsureDocEntry()
 {
   if (!mDocEntry && nsContentUtils::IsPerformanceNavigationTimingEnabled()) {
     nsCOMPtr<nsIHttpChannel> httpChannel = do_QueryInterface(mChannel);
-    mDocEntry = new PerformanceNavigationTiming(Timing(), this,
+    RefPtr<PerformanceTiming> timing =
+      new PerformanceTiming(this, mChannel, nullptr, 0);
+    mDocEntry = new PerformanceNavigationTiming(timing, this,
                                                 httpChannel);
   }
 }

dom/performance/PerformanceNavigationTiming.h

@@ -24,6 +24,10 @@ class PerformanceNavigationTiming final
 public:
   NS_DECL_ISUPPORTS_INHERITED
 
+  // Note that aPerformanceTiming must be initalized with zeroTime = 0
+  // so that timestamps are relative to startTime, as opposed to the
+  // performance.timing object for which timestamps are absolute and has a
+  // zeroTime initialized to navigationStart
   explicit PerformanceNavigationTiming(PerformanceTiming* aPerformanceTiming,
                                        Performance* aPerformance,
                                        nsIHttpChannel* aChannel)