WebSocketStream: Use a hwm of 1 for the readable

Initially WebSocketStream used a hwm of 0 for the ReadableStream that is
returned from the `connected` promise, but that resulted in poor
throughput when the application wasn't continuously reading.

Change the hwm of 1 so that up to one message will still be transferred
to the render process even when it is not inside a read() call.
Backpressure will still be applied if more than 1 message is in transit.

BUG=983030

Change-Id: I883d9e62b8465d45bc9b383db8f639fbf791c78d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2054605Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742029}

third_party/blink/renderer/modules/websockets/websocket_channel_impl.cc

@@ -424,12 +424,16 @@ void WebSocketChannelImpl::CancelHandshake() {
 }
 
 void WebSocketChannelImpl::ApplyBackpressure() {
+  NETWORK_DVLOG(1) << this << " ApplyBackpressure";
   backpressure_ = true;
 }
 
 void WebSocketChannelImpl::RemoveBackpressure() {
+  NETWORK_DVLOG(1) << this << " RemoveBackpressure";
+  if (backpressure_) {
     backpressure_ = false;
     ConsumePendingDataFrames();
+  }
 }
 
 void WebSocketChannelImpl::OnOpeningHandshakeStarted(

third_party/blink/renderer/modules/websockets/websocket_stream.cc

@@ -485,7 +485,7 @@ void WebSocketStream::DidConnect(const String& subprotocol,
   connection->setExtensions(extensions);
   source_ = MakeGarbageCollected<UnderlyingSource>(script_state_, this);
   auto* readable = ReadableStream::CreateWithCountQueueingStrategy(
-      script_state_, source_, 0);
+      script_state_, source_, 1);
   sink_ = MakeGarbageCollected<UnderlyingSink>(this);
   auto* writable =
       WritableStream::CreateWithCountQueueingStrategy(script_state_, sink_, 1);

third_party/blink/web_tests/external/wpt/websockets/handlers/send-backpressure_wsh.py

@@ -19,8 +19,15 @@ def web_socket_do_extra_handshake(request):
 
 
 def web_socket_transfer_data(request):
+    # Send empty message to fill the ReadableStream queue
+    request.ws_stream.send_message(b'', binary=True)
+
     # TODO(ricea@chromium.org): Use time.perf_counter() when migration to python
     # 3 is complete. time.time() can go backwards.
     start_time = time.time()
+
+    # The large message that will be blocked by backpressure.
     request.ws_stream.send_message(b' ' * MESSAGE_SIZE, binary=True)
+
+    # Report the time taken to send the large message.
     request.ws_stream.send_message(six.text_type(time.time() - start_time), binary=False)

third_party/blink/web_tests/external/wpt/websockets/stream-tentative/backpressure-receive.any.js

@@ -19,6 +19,9 @@ promise_test(async t => {
   // Create backpressure for 2 seconds.
   await new Promise(resolve => t.step_timeout(resolve, 2000));
 
+  // Skip the empty message used to fill the readable queue.
+  await reader.read();
+
   // Skip the large message.
   await reader.read();