Add components/keyed_service to GN build.

components/keyed_service/core/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.
+
+component("keyed_service_core") {
+  output_name = "keyed_service_core"
+  sources = [
+    "dependency_graph.cc",
+    "dependency_graph.h",
+    "dependency_node.h",
+    "keyed_service.cc",
+    "keyed_service.h",
+    "keyed_service_export.h",
+  ]
+
+  defines = [ "KEYED_SERVICE_IMPLEMENTATION" ]
+  if (is_win) {
+    # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
+    cflags = [ "/wd4267" ]
+  }
+
+  deps = [
+    "//base",
+  ]
+}