Fix and re-enable TCPSocketTest.ServerReceivesMultipleAccept

This test is flaky because the test closes client connection during each
for-loop iteration. When server checks whether the connection is still connected
using accepted_socket_->GetPeerAddress() in
network::TCPServerSocket::OnAcceptCompleted(), it will get
net::ERR_SOCKET_NOT_CONNECTED (-15).

This test keeps the client sockets alive and delay disconnecting them until
server finishes all Accept() callbacks.

Bug: 821224
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I7bf12704272f9ccc23c085c6d72db28a3d0dbe67
Reviewed-on: https://chromium-review.googlesource.com/966802Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Helen Li <xunjieli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543743}

services/network/tcp_socket_unittest.cc

@@ -477,8 +477,7 @@ TEST_F(TCPSocketTest, CannotConnectToWrongInterface) {
   }
 }
 
-// TODO(xunjieli): This test is flaky on MacOS crbug.com/821224.
-TEST_F(TCPSocketTest, DISABLED_ServerReceivesMultipleAccept) {
+TEST_F(TCPSocketTest, ServerReceivesMultipleAccept) {
   uint32_t backlog = 10;
   TestServer server;
   server.Start(backlog);
@@ -496,6 +495,7 @@ TEST_F(TCPSocketTest, DISABLED_ServerReceivesMultipleAccept) {
   EXPECT_EQ(net::ERR_INSUFFICIENT_RESOURCES, callback.WaitForResult());
 
   // After handling incoming connections, all callbacks should now complete.
+  std::vector<mojom::TCPConnectedSocketPtr> client_sockets;
   for (size_t i = 0; i < backlog; ++i) {
     TestTCPConnectedSocketObserver observer;
     mojo::ScopedDataPipeConsumerHandle client_socket_receive_handle;
@@ -506,6 +506,7 @@ TEST_F(TCPSocketTest, DISABLED_ServerReceivesMultipleAccept) {
                   mojo::MakeRequest(&client_socket), observer.GetObserverPtr(),
                   base::nullopt /*local_addr*/, server.server_addr(),
                   &client_socket_receive_handle, &client_socket_send_handle));
+    client_sockets.push_back(std::move(client_socket));
   }
   for (const auto& callback : accept_callbacks) {
     EXPECT_EQ(net::OK, callback->WaitForResult());