Use datagram queue in network::QuicTransport

This way, the datagrams have a small time-limited buffer that allows
them to work nicely with pacing.

R=yhirano@chromium.org

Bug: 1011392
Change-Id: Iad925e4755d550ef352901145b651eff70e35228
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2001233Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Victor Vasiliev <vasilvv@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732122}

services/network/quic_transport.cc

@@ -278,9 +278,10 @@ void QuicTransport::SendDatagram(base::span<const uint8_t> data,
   memcpy(buffer->data(), data.data(), data.size());
   quic::QuicMemSlice slice(
       quic::QuicMemSliceImpl(std::move(buffer), data.size()));
-  const quic::MessageResult result =
-      transport_->session()->SendMessage(quic::QuicMemSliceSpan(&slice));
-  std::move(callback).Run(result.status == quic::MESSAGE_STATUS_SUCCESS);
+  const quic::MessageStatus status =
+      transport_->session()->datagram_queue()->SendOrQueueDatagram(
+          std::move(slice));
+  std::move(callback).Run(status == quic::MESSAGE_STATUS_SUCCESS);
 }
 
 void QuicTransport::CreateStream(