Migrate to zx_socket_shutdown() API to close the socket for writing.

Zircon previously used an option to zx_socket_write() to close the
socket for reading, or writing. Migrate to the new zx_socket_shutdown()
API.

Change-Id: Ia4067c4611713031048271800dd620a9c1b15e72
Reviewed-on: https://chromium-review.googlesource.com/c/1334329Reviewed-by: Wez <wez@chromium.org>
Reviewed-by: Adam Barth <abarth@chromium.org>
Reviewed-by: Fabrice de Gans-Riberi <fdegans@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607774}

webrunner/net_http/url_loader_impl.cc

@@ -340,7 +340,7 @@ bool URLLoaderImpl::WriteResponseBytes(int result) {
              response_body_mode_ ==
                  oldhttp::ResponseBodyMode::BUFFER_OR_STREAM);
       // In socket mode, attempt to shut down the socket and close it.
-      write_socket_.write(ZX_SOCKET_SHUTDOWN_WRITE, nullptr, 0, nullptr);
+      write_socket_.shutdown(ZX_SOCKET_SHUTDOWN_WRITE);
       write_socket_ = zx::socket();
     } else {
       DCHECK_EQ(response_body_mode_, oldhttp::ResponseBodyMode::BUFFER);