Move enable_mdns build flag to net.

Moves it out of the global feature flags and into the net feature flags.
Migrates the flag from a #define to a buildflag.

Review-Url: https://codereview.chromium.org/2460653003
Cr-Original-Commit-Position: refs/heads/master@{#428514}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 5224a189c5968ac267a0c5456846c81495e7dfe3

config/BUILD.gn

@@ -62,9 +62,6 @@ declare_args() {
 config("feature_flags") {
   # Don't use deprecated V8 APIs anywhere.
   defines = [ "V8_DEPRECATION_WARNINGS" ]
-  if (enable_mdns) {
-    defines += [ "ENABLE_MDNS=1" ]
-  }
   if (enable_notifications) {
     defines += [ "ENABLE_NOTIFICATIONS" ]
   }

config/features.gni

@@ -21,9 +21,6 @@ if (is_android) {
 }
 
 declare_args() {
-  # Multicast DNS.
-  enable_mdns = is_win || is_linux
-
   enable_extensions = !is_android && !is_ios
   enable_plugins = (!is_android && !is_ios) || is_chromecast
   enable_pdf = !is_android && !is_ios && !is_chromecast