Add option to allow mojob to specify extra gn args.

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 e584e1f41b1dd30dfd74249f817efc640c71fd48..83f095891f0ed448316fe3e99ff18315c7086cc9 100644
--- a/mojo/public/tools/BUILD.gn
+++ b/mojo/public/tools/BUILD.gn
@@ -31,16 +31,26 @@ if (use_prebuilt_mojo_shell) {
 if (use_prebuilt_network_service) {
   copy("copy_network_service") {
     filename = "network_service.mojo"
-    if (is_android) {
+    if (defined(self_built_network_service_location) &&

[qsr, 2015/03/09 11:58:41]

Don't you need to define this variable somewhere?

Also, maybe name it prebuilt_network_service_location instead.

[haltonhuo, 2015/03/09 12:13:00]

As blundell suggested, I move the declaration in build/module_args/mojo.gni,
which is changed in CL https://codereview.chromium.org/983143003/

I'm okay with the name "prebuilt_network_service_location", will change in next
patch set.

[haltonhuo, 2015/03/10 06:50:43]

Done.

+        self_built_network_service_location != "") {
       sources = [
-        "prebuilt/network_service/android-arm/$filename",
+        "$self_built_network_service_location",
       ]
     } else {
-      assert(is_linux)
-      sources = [
-        "prebuilt/network_service/linux-x64/$filename",
-      ]
+      if (is_android) {
+        assert(cpu_arch == "arm",
+               "Only arm version of prebuilt netowrk_service.mojo available.")
+        sources = [
+          "prebuilt/network_service/android-arm/$filename",
+        ]
+      } else {
+        assert(is_linux)
+        sources = [
+          "prebuilt/network_service/linux-x64/$filename",
+        ]
+      }
     }
+
     outputs = [
       "$root_out_dir/$filename",
     ]