[ios] Adds test hooks to enable/disable functionality while testing.

ios/chrome/app/tests_fake_hook.mm

+// Copyright 2016 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 "ios/chrome/app/tests_hook.h"
+
+namespace tests_hook {
+
+bool DisableContextualSearch() {
+  return false;
+}
+bool DisableFirstRun() {
+  return false;
+}
+bool DisableSigninRecallPromo() {
+  return false;
+}
+bool DisableUpdateService() {
+  return false;
+}
+void SetUpTestsIfPresent() {}
+void RunTestsIfPresent() {}
+
+}  // namespace tests_hook