Add BUILD.gn to build ui strings from grit.

ui/base/strings/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("//tools/grit/grit_rule.gni")
+
+# Meta target that includes both ui_strings and app_locale_settings. Most
+# targets want both. You can depend on the individually if you need to.
+group("strings") {
+  deps = [
+    ":ui_strings",
+    ":app_locale_settings",
+  ]
+}
+
+grit("ui_strings") {
+  source = "ui_strings.grd"
+}
+
+grit("app_locale_settings") {
+  source = "app_locale_settings.grd"
+}