Add support for building "ios_chrome_unittests" with gn.

ios/chrome/browser/variations/BUILD.gn

+# Copyright 2015 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("//components/variations/service/generate_ui_string_overrider.gni")
+
+generate_ui_string_overrider("ios_chrome_ui_string_overrider_factory") {
+  inputs = [
+    "$root_gen_dir/components/strings/grit/components_chromium_strings.h",
+    "$root_gen_dir/components/strings/grit/components_google_chrome_strings.h",
+    "$root_gen_dir/components/strings/grit/components_strings.h",
+    "$root_gen_dir/ios/chrome/grit/ios_chromium_strings.h",
+    "$root_gen_dir/ios/chrome/grit/ios_google_chrome_strings.h",
+    "$root_gen_dir/ios/chrome/grit/ios_locale_settings.h",
+    "$root_gen_dir/ios/chrome/grit/ios_strings.h",
+  ]
+  deps = [
+    "//components/strings:components_chromium_strings",
+    "//components/strings:components_google_chrome_strings",
+    "//components/strings:components_strings",
+    "//ios/chrome/app/strings:ios_chromium_strings",
+    "//ios/chrome/app/strings:ios_google_chrome_strings",
+    "//ios/chrome/app/strings:ios_locale_settings",
+    "//ios/chrome/app/strings:ios_strings",
+  ]
+  namespace = ""  # Put the functions in the global namespace on iOS.
+  header_filename = "ios_ui_string_overrider_factory.h"
+  source_filename = "ios_ui_string_overrider_factory.cc"
+}