Disabled network service feature.

When enabled this feature does not instantiate ChromeNetworkDelegate and
by its virtue BraveNetworkDelegateBase which is where we add hooks into
network stack.

Related: brave/brave-browser#2351

Chromium change to not instantiate ChromeNetworkDelegate:

commit  4b7d02f22b70c27f32fa5508f785ba310d08aad3
Author: John Abd-El-Malek < [email protected]  >
Date: Thu May 03 17:46:35 2018

Don't instantiate ChromeNetworkDelegate when network service is enabled.

Bug:  598073

Chromium change in C74 to enable network service by default:

commit 708db9bc46bec1d0037f44f8fdaff7b7d72724e0
Author: John Abd-El-Malek <[email protected]>
Date:   Mon Feb 25 18:10:35 2019 +0000

    Enable network service on Win/Mac/Linux by default as it launched on stable.

    Removing running integration tests on these bots (waterfall + CQ).

    Keep running the layout tests with network service disabled on on debug Linux Tests bots as we're still supporting ChromeOS/Android/Cast which haven't switched over yet.

    Also remove the disabled-service-worker-servicification virtual test.

    Bug: 933880,926114

app/brave_main_delegate.cc

@@ -30,6 +30,7 @@
 #include "content/public/common/content_features.h"
 #include "extensions/common/extension_features.h"
 #include "gpu/config/gpu_finch_features.h"
+#include "services/network/public/cpp/features.h"
 #include "third_party/widevine/cdm/buildflags.h"
 #include "ui/base/ui_base_features.h"
 
@@ -147,6 +148,7 @@ bool BraveMainDelegate::BasicStartupComplete(int* exit_code) {
       features::kDefaultEnableOopRasterization.name,
       features::kSharedArrayBuffer.name,
       features::kWebAssemblyThreads.name,
+      network::features::kNetworkService.name,
       unified_consent::kUnifiedConsent.name,
   };