Expose ability to download network service impl in SDK.

mojo/public/mojo.gni

Index: mojo/public/mojo.gni
diff --git a/mojo/public/mojo.gni b/mojo/public/mojo.gni
index 9f1b86b7d203dab69125d1c54555de3066e831d0..23aac005706ae2779f9303419b7555a6fa8f0397 100644
--- a/mojo/public/mojo.gni
+++ b/mojo/public/mojo.gni
@@ -10,6 +10,12 @@ if (!defined(use_prebuilt_mojo_shell) || use_prebuilt_mojo_shell) {
   use_prebuilt_mojo_shell = is_linux || is_android
 }
 
+# If using the prebuilt network service, gate its usage by the platforms for
+# which it is published.
+if (!defined(use_prebuilt_network_service) || use_prebuilt_network_service) {
+  use_prebuilt_network_service = is_linux || is_android
+}
+
 # The absolute path to the directory containing the mojo public SDK (i.e., the
 # directory containing mojo/public). The build files within the Mojo public
 # SDK use this variable to allow themselves to be parameterized by the location