WebGPU CTS: Send CONNECTION_ACK when the websocket is open

Bug: chromium:1434762
Change-Id: Id287588565356eca8222a0ef43a35619a1f6f8b1
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/131360
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Loko Kung <lokokung@google.com>
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Brian Sheedy <bsheedy@google.com>

webgpu-cts/test_runner.js

@@ -91,6 +91,9 @@ function byteSize(s) {
 
 async function setupWebsocket(port) {
   socket = new WebSocket('ws://127.0.0.1:' + port)
+  socket.addEventListener('open', () => {
+    socket.send('{"type":"CONNECTION_ACK"}');
+  });
   socket.addEventListener('message', runCtsTestViaSocket);
 }