Add TODO to //mojo/network/BUILD.gn

This file needs to be flexible wrt the location of the Mojo SDK only for the
duration of the time that it is being used to build the network service impl in
Mojo as well as in Chromium.

Review URL: https://codereview.chromium.org/871463002

Cr-Commit-Position: refs/heads/master@{#312799}

mojo/services/network/BUILD.gn

@@ -2,6 +2,9 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+# TODO(ppi): Switch this file back to using absolute paths to refer to the Mojo
+# SDK once it is no longer being used to build the network service impl in
+# Mojo.
 import("../mojo_sdk_root.gni")
 import("$mojo_sdk_root/mojo/public/mojo_application.gni")