Network service: start the process as early as possible.

That significantly improves the startup performance: if a URL is loaded using
commandline flag, starting the network service process early could reduce the
timeToOnLoad number (from navigation start to onload event) by 0.5 ~ 1 second.

BUG=598073

Review-Url: https://codereview.chromium.org/2854713002
Cr-Commit-Position: refs/heads/master@{#468496}

content/browser/service_manager/service_manager_context.cc

@@ -357,6 +357,11 @@ ServiceManagerContext::ServiceManagerContext() {
 
   packaged_services_connection_->Start();
   ServiceManagerConnection::GetForProcess()->Start();
+
+  // Start the network service process as soon as possible, since it is critical
+  // to start up performance.
+  ServiceManagerConnection::GetForProcess()->GetConnector()->StartService(
+      mojom::kNetworkServiceName);
 }
 
 ServiceManagerContext::~ServiceManagerContext() {