Make //mojo/services/network/interfaces/BUILD.gn reusable by consumers.

Turns out all we needed is mojo_sdk_root, which
//build/module_args/mojo.gni already has.

R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/1399883007 .
diff --git a/mojo/services/network/interfaces/BUILD.gn b/mojo/services/network/interfaces/BUILD.gn
index 2c00d50..819eb82 100644
--- a/mojo/services/network/interfaces/BUILD.gn
+++ b/mojo/services/network/interfaces/BUILD.gn
@@ -3,7 +3,7 @@
 # found in the LICENSE file.
 
 import("//build/module_args/mojo.gni")
-import("//third_party/mojo/src/mojo/public/tools/bindings/mojom.gni")
+import("$mojo_sdk_root/mojo/public/tools/bindings/mojom.gni")
 
 mojom("interfaces") {
   sources = [
@@ -23,12 +23,10 @@
     "web_socket.mojom",
   ]
 
-  deps = [
-    "//third_party/mojo/src/mojo/public/interfaces/network",
-  ]
+  mojo_sdk_deps = [ "mojo/public/interfaces/network" ]
 
   import_dirs = [
     get_path_info("../../", "abspath"),
-    "//third_party/mojo/src",
+    "$mojo_sdk_root",
   ]
 }