Add more GN deps to the build.

components/policy/proto/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("//third_party/protobuf/proto_library.gni")
+
+proto_library("proto") {
+  sources = [
+    "device_management_backend.proto",
+    "policy_signing_key.proto",
+  ]
+  if (!is_android && !is_ios) {
+    sources += [ "chrome_extension_policy.proto" ]
+  }
+  if (is_chromeos) {
+    sources += [ "device_management_local.proto" ]
+  }
+
+  cc_generator_options = "dllexport_decl=POLICY_PROTO_EXPORT:"
+  cc_include = "components/policy/policy_proto_export.h"
+  defines = [ "POLICY_PROTO_COMPILATION" ]
+}