Convert net feature defines to a buildflag header.

There were 4 net-only flags defined by src/net that were defined in a config
and passed on the command line. This converts these flags to the buildflag
system for better scalability and maintainability.

Review-Url: https://codereview.chromium.org/2450183005
Cr-Commit-Position: refs/heads/master@{#428276}
diff --git a/chrome/browser/io_thread.cc b/chrome/browser/io_thread.cc
index a6c94b73..6e81a66 100644
--- a/chrome/browser/io_thread.cc
+++ b/chrome/browser/io_thread.cc
@@ -88,6 +88,7 @@
 #include "net/http/http_auth_preferences.h"
 #include "net/http/http_network_layer.h"
 #include "net/http/http_server_properties_impl.h"
+#include "net/net_features.h"
 #include "net/nqe/external_estimate_provider.h"
 #include "net/nqe/network_quality_estimator.h"
 #include "net/proxy/proxy_config_service.h"
@@ -1060,7 +1061,7 @@
           content::BrowserThread::GetBlockingPool()
               ->GetTaskRunnerWithShutdownBehavior(
                   base::SequencedWorkerPool::SKIP_ON_SHUTDOWN)));
-#if !defined(DISABLE_FTP_SUPPORT)
+#if !BUILDFLAG(DISABLE_FTP_SUPPORT)
   job_factory->SetProtocolHandler(
       url::kFtpScheme,
       net::FtpProtocolHandler::Create(globals->host_resolver.get()));