Bug 1376449 - Robustify startDrawing to prevent calling callback function after stop() has been called. r=pehrsons

MozReview-Commit-ID: 5vPtTHBGJhN

--HG--
extra : rebase_source : fc5ac99c261120c10d43616650c94bdbf63d3bbc

dom/canvas/test/captureStream_common.js

@@ -42,8 +42,9 @@ CaptureStreamTestHelper.prototype = {
   startDrawing: function (f) {
     var stop = false;
     var draw = () => {
+      if (stop) { return; }
       f();
-      if (!stop) { window.requestAnimationFrame(draw); }
+      window.requestAnimationFrame(draw);
     };
     draw();
     return { stop: () => stop = true };