Bug 1436778 Add an assertion to hopefully get a reproduction for a hard-to-repro timer jitter bug r=baku

MozReview-Commit-ID: D4zt1v1tjOs

--HG--
extra : rebase_source : b4f5be226c72d6a2f80b4f598155fe9f11ee695c

dom/performance/PerformanceTiming.cpp

@@ -86,6 +86,15 @@ PerformanceTiming::PerformanceTiming(Performance* aPerformance,
     : nsRFPService::ReduceTimePrecisionAsMSecs(aZeroTime,
         aPerformance->GetRandomTimelineSeed())));
 
+#ifdef DEBUG
+  if (mTimingData->ResponseStartHighRes(aPerformance) - mTimingData->ZeroTime() < 0) {
+    MOZ_CRASH_UNSAFE_PRINTF("Heisenbug Reproduced: Please file line in 1436778. %s %f - %f (%f)",
+      (aPerformance->IsSystemPrincipal() ? "System" : "Not-System"),
+      mTimingData->ResponseStartHighRes(aPerformance),
+      mTimingData->ZeroTime(), aZeroTime);
+  }
+#endif
+
   // Non-null aHttpChannel implies that this PerformanceTiming object is being
   // used for subresources, which is irrelevant to this probe.
   if (!aHttpChannel &&