Bug 1467920, ensure only non-negative values are reported to telemetry, r=mccr8

--HG--
extra : rebase_source : 09a804d5a4ee753c32a09c79c7bebe1ed85a5713

dom/base/nsJSEnvironment.cpp

@@ -1281,7 +1281,9 @@ FireForgetSkippable(uint32_t aSuspected, bool aRemoveChildless,
     if (!aDeadline.IsNull()) {
       if (aDeadline < now) {
         // This slice overflowed the idle period.
-        idleDuration = aDeadline - startTimeStamp;
+        if (aDeadline > startTimeStamp) {
+          idleDuration = aDeadline - startTimeStamp;
+        }
       } else {
         idleDuration = duration;
       }