Specify mojo_prebuilt_network_service_apptests_location via mojob.py.

mojo/public/tools/BUILD.gn

Index: mojo/public/tools/BUILD.gn
diff --git a/mojo/public/tools/BUILD.gn b/mojo/public/tools/BUILD.gn
index 9344cf31027420146cee303312892347276493fd..8b47bb91dacc98485f2e3277c3ba5ccc3688089f 100644
--- a/mojo/public/tools/BUILD.gn
+++ b/mojo/public/tools/BUILD.gn
@@ -58,15 +58,25 @@ if (mojo_use_prebuilt_network_service) {
 
   copy("copy_network_service_apptests") {
     filename = "network_service_apptests.mojo"
-    if (is_android) {
+    if (defined(mojo_prebuilt_network_service_apptests_location) &&
+        mojo_prebuilt_network_service_apptests_location != "") {
       sources = [
-        "prebuilt/network_service_apptests/android-arm/$filename",
+        "$mojo_prebuilt_network_service_apptests_location",
       ]
     } else {
-      assert(is_linux)
-      sources = [
-        "prebuilt/network_service_apptests/linux-x64/$filename",
-      ]
+      if (is_android) {
+        assert(
+            cpu_arch == "arm",
+            "Only arm version prebuilt netowrk_service_apptests.mojo is available.")
+        sources = [
+          "prebuilt/network_service_apptests/android-arm/$filename",
+        ]
+      } else {
+        assert(is_linux)
+        sources = [
+          "prebuilt/network_service_apptests/linux-x64/$filename",
+        ]
+      }
     }
     outputs = [
       "$root_out_dir/$filename",