Make more services' public interfaces flexible wrt location in repo.

mojo/services/window_manager/public/interfaces/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("../../../mojo_sdk_root.gni")
 import("$mojo_sdk_root/mojo/public/tools/bindings/mojom.gni")
 
 mojom("interfaces") {
   sources = [
     "window_manager.mojom",
     "window_manager_internal.mojom",
   ]
 
+  import_dirs = [ get_path_info("../../../", "abspath") ]
+
   mojo_sdk_deps = [ "mojo/public/interfaces/application" ]
 
   deps = [
     "//mojo/services/geometry/public/interfaces",
     "//mojo/services/input_events/public/interfaces",
   ]
 }