Bug 1430173 - Reduce the precision of all explicit clocks to 2ms. r=baku, a=RyanVM

Backport to ESR where we don't have the ResistFingerprinting component.

MozReview-Commit-ID: 9bjycHjR3SF

--HG--
extra : transplant_source : %EA%03%21%0A%E9%3F%8E%CD%7C%D79f%96%85%96%00%5D%7F%95X

dom/performance/Performance.cpp

@@ -21,6 +21,7 @@
 #include "mozilla/dom/PerformanceObserverBinding.h"
 #include "mozilla/IntegerPrintfMacros.h"
 #include "mozilla/Preferences.h"
+#include "mozilla/TimerClamping.h"
 #include "WorkerPrivate.h"
 #include "WorkerRunnable.h"
 
@@ -228,9 +229,9 @@ Performance::ClearResourceTimings()
 DOMHighResTimeStamp
 Performance::RoundTime(double aTime) const
 {
-  // Round down to the nearest 20us, because if the timer is too accurate people
+  // Round down to the nearest 2ms, because if the timer is too accurate people
   // can do nasty timing attacks with it.
-  const double maxResolutionMs = 0.020;
+  const double maxResolutionMs = 2;
   return floor(aTime / maxResolutionMs) * maxResolutionMs;
 }