Bug 1430173 - Add Timer Rounding tests backported from -central to -esr. r=baku, a=RyanVM

MozReview-Commit-ID: Jl4WZAamgrI

--HG--
extra : transplant_source : E%DC%91lU%1C%A4l%2C%C8%23PCz%EB%F2%81%25%1F%90

dom/tests/mochitest/general/worker_grandchild.js

@@ -0,0 +1,10 @@
+self.addEventListener("message", function(event) {
+  let timeStampCodes = event.data;
+
+  let timeStamps = [];
+  for (let timeStampCode of timeStampCodes) {
+    timeStamps.push(eval(timeStampCode));
+  }
+  // Send the timeStamps to the parent.
+  postMessage(timeStamps);
+});