Send video bitrate and user bandwidth to Watchman (#7145)

* adding functionality to detect user download speed

* calculating bandwidth speed more intelligently

* saving download speed and updating it every 30s

* all the functionality should be done needs testing

* fix linting

* use a 1mb file for calculating bandwidth

* add optional chaining plugin to babel and get bitrate from texttrack

* allow optional chaining for flow

* ignore flow error

* disable bandwidth checking functionality

* fix flow error

ui/util/detect-user-bandwidth.js

@@ -0,0 +1,20 @@
+const imageAddr = 'https://upload.wikimedia.org/wikipedia/commons/b/b9/Pizigani_1367_Chart_1MB.jpg';
+const downloadSize = 1093957; // this must match with the image above
+
+let startTime, endTime;
+async function measureConnectionSpeed() {
+  startTime = (new Date()).getTime();
+  const cacheBuster = '?nnn=' + startTime;
+
+  const download = new Image();
+  download.src = imageAddr + cacheBuster;
+  // this returns when the image is finished downloading
+  await download.decode();
+  endTime = (new Date()).getTime();
+  const duration = (endTime - startTime) / 1000;
+  const bitsLoaded = downloadSize * 8;
+  const speedBps = (bitsLoaded / duration).toFixed(2);
+  return Math.round(Number(speedBps));
+}
+
+module.exports = measureConnectionSpeed;