Enable websocket filtering via SubresourceFilter

chrome/test/data/subresource_filter/page_with_websocket.html

+<script src='websocket_connection.js'></script>
+<script>
+// Listens for 'onmessage' or 'onclose' messages from the websocket, and sends
+// them to the test fixture.
+function connectionListener(e) {
+  if (e.data == 'onmessage') {
+    window.domAutomationController.send(true);
+  } else if (e.data == 'onclose') {
+    window.domAutomationController.send(false);
+  }
+}
+
+// This page has two modes, one where the websocket connection lives in the
+// document context, and one where it lives in a worker. This is encoded by the
+// extra 'inWorker' url param.

[engedy, 2017/03/06 16:31:01]

nit: presence (or absence) of an URL param named `inWorker`.

[Charlie Harrison, 2017/03/06 19:22:11]

Done.

+var url = new URL(location);
+if (url.searchParams.get('inWorker')) {
+  var worker = new Worker('websocket_connection.js');
+  worker.addEventListener('message', connectionListener);
+
+  // Redefine connectWebSocket to instead post a message to start the worker.
+  connectWebSocket = function(url) {
+    worker.postMessage({url: url});
+  }
+} else {
+  addEventListener('message', connectionListener);
+}
+</script>