Fix test that deleting the primary NetworkContext destroys all others.

The test wasn't waiting for the secondary NetworkContext to be created,
so it could end up that the secondary NetworkContext was created only
after the primary one was destroyed.

Bug: 869551
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: Iaefbb98553175cf4e3784017adafe227888c588b
Reviewed-on: https://chromium-review.googlesource.com/1159588Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580127}

services/network/network_service_unittest.cc

@@ -991,6 +991,10 @@ TEST_F(NetworkServiceTestWithService,
                                          CreateContextParams());
   network_context.set_connection_error_handler(run_loop.QuitClosure());
 
+  // Wait until the new NetworkContext has been created, so it's not created
+  // after the primary NetworkContext is destroyed.
+  network_service_.FlushForTesting();
+
   // Destroying |cert_validating_network_context| should result in destroying
   // |network_context| as well.
   cert_validating_network_context.reset();