Bug 1188616 - CPOW monitoring should use JS_Now instead of PR_IntervalNow. r=jandem

--HG--
extra : transplant_source : %B9h%9D%1B%CF%19%D5%D9%D2%BDo.%3D%D9%EB%0E%03%EF%95%B7

js/ipc/CPOWTimer.cpp

@@ -9,6 +9,8 @@
 #include "nsContentUtils.h"
 #include "CPOWTimer.h"
 
+#include "jsapi.h"
+
 CPOWTimer::CPOWTimer(JSContext* cx MOZ_GUARD_OBJECT_NOTIFIER_PARAM_IN_IMPL)
     : cx_(nullptr)
     , startInterval_(0)
@@ -18,7 +20,7 @@ CPOWTimer::CPOWTimer(JSContext* cx MOZ_GUARD_OBJECT_NOTIFIER_PARAM_IN_IMPL)
     if (!js::GetStopwatchIsMonitoringCPOW(runtime))
         return;
     cx_ = cx;
-    startInterval_ = PR_IntervalNow();
+    startInterval_ = PR_Now();
 }
 CPOWTimer::~CPOWTimer()
 {
@@ -33,7 +35,12 @@ CPOWTimer::~CPOWTimer()
         return;
     }
 
+    const int64_t endInterval = PR_Now();
+    if (endInterval <= startInterval_) {
+        // Do not assume monotonicity.
+        return;
+    }
+
     js::PerformanceData* performance = js::GetPerformanceData(runtime);
-    uint64_t duration = PR_IntervalToMicroseconds(PR_IntervalNow() - startInterval_);
-    performance->totalCPOWTime += duration;
+    performance->totalCPOWTime += endInterval - startInterval_;
 }

js/ipc/CPOWTimer.h

@@ -38,7 +38,7 @@ class MOZ_STACK_CLASS CPOWTimer final {
      * The instant at which the stopwatch was started. Undefined
      * if CPOW monitoring was off when the timer was created.
      */
-    PRIntervalTime startInterval_;
+    int64_t startInterval_;
 };
 
 #endif