Prefix build variables used in separate repos with mojo_

As requested by Brett here:
https://codereview.chromium.org/1029113005/diff/20001/build/module_args/mojo.gni

R=blundell@chromium.org, viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/1034603002
diff --git a/mojo/BUILD.gn b/mojo/BUILD.gn
index ca646cd..18d0158 100644
--- a/mojo/BUILD.gn
+++ b/mojo/BUILD.gn
@@ -79,7 +79,7 @@
     deps += [ "//services/python:python_apptests" ]
   }
 
-  if (use_prebuilt_network_service) {
+  if (mojo_use_prebuilt_network_service) {
     deps += [ "//mojo/public/tools:copy_network_service_apptests" ]
   }
 }