e2ee: detect transferable streams

Since we use transferable streams to transfer the streams to the worker
we need to feature detect them as Chrome 86 will launch Insertable Streams
by default without shipping transferable streams yet (status still unclear,
it might still ship too)

See here for the feature detection used:
  https://groups.google.com/a/chromium.org/g/blink-dev/c/1LStSgBt6AM/m/hj0odB8pCAAJ

modules/browser/BrowserCapabilities.js

@@ -281,9 +281,23 @@ export default class BrowserCapabilities extends BrowserDetection {
      * @returns {boolean} {@code true} if the browser supports insertable streams.
      */
     supportsInsertableStreams() {
-        return Boolean(typeof window.RTCRtpSender !== 'undefined'
+        if (!(typeof window.RTCRtpSender !== 'undefined'
             && (window.RTCRtpSender.prototype.createEncodedStreams
-                || window.RTCRtpSender.prototype.createEncodedVideoStreams));
+                || window.RTCRtpSender.prototype.createEncodedVideoStreams))) {
+            return false;
+        }
+
+        // Feature-detect transferable streams which we need to operate in a worker.
+        // See https://groups.google.com/a/chromium.org/g/blink-dev/c/1LStSgBt6AM/m/hj0odB8pCAAJ
+        const stream = new ReadableStream();
+
+        try {
+            window.postMessage(stream, '*', [ stream ]);
+
+            return true;
+        } catch {
+            return false;
+        }
     }
 
     /**