Add a feature flag for network service and about:flags entry for it.

content/public/common/content_switches.cc

Index: content/public/common/content_switches.cc
diff --git a/content/public/common/content_switches.cc b/content/public/common/content_switches.cc
index a7aef8bab7573292183681d013173463d73de006..6a0e42300df673011cfbb61cab2a48cfb99334f9 100644
--- a/content/public/common/content_switches.cc
+++ b/content/public/common/content_switches.cc
@@ -403,9 +403,6 @@ const char kEnableLogging[]                 = "enable-logging";
 // Enables the network information API.
 const char kEnableNetworkInformation[]      = "enable-network-information";
 
-// Enables the network service.
-const char kEnableNetworkService[] = "enable-network-service";
-
 // Disables the video decoder from drawing to an NV12 textures instead of ARGB.
 const char kDisableNv12DxgiVideo[] = "disable-nv12-dxgi-video";