Support of specify local prebuilt network service apptests file.

Same as https://codereview.chromium.org/983143003, we need specify
network_service_apptests.mojo as well.

BUG=472609

Review URL: https://codereview.chromium.org/1048413002

Cr-Original-Commit-Position: refs/heads/master@{#323656}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 78d4a7e127b51a315bbb0b26aadd7e601a50e50f

module_args/mojo.gni

@@ -29,4 +29,7 @@ declare_args() {
   # android_x64. And it is also useful for supportted platform as well when
   # developer want to try its own version of network service files.
   mojo_prebuilt_network_service_location = ""
+
+  # For network_service_apptests.mojo
+  mojo_prebuilt_network_service_apptests_location = ""
 }