Add Desktop iOS promotion logging to chrome ios app.

ios/chrome/browser/desktop_promotion/desktop_promotion_prefs.h

+// Copyright 2017 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.
+
+#ifndef CHROME_BROWSER_DESKTOP_PROMOTION_DESKTOP_PROMOTION_PREFS_H
+#define CHROME_BROWSER_DESKTOP_PROMOTION_DESKTOP_PROMOTION_PREFS_H
+
+namespace user_prefs {
+class PrefRegistrySyncable;
+}
+
+namespace prefs {
+extern const char kDesktopIOSPromotionSMSEntryPoint[];
+extern const char kDesktopIOSPromotionTargetedEntryPoints[];
+extern const char kDesktopIOSPromotionLastInteraction[];
+extern const char kDesktopIOSPromotionDone[];
+}  // namespace prefs
+
+namespace desktop_ios_promotion {
+
+// Register profile specific desktop promotion related preferences.
+void RegisterDesktopPromotionUserPrefs(
+    user_prefs::PrefRegistrySyncable* registry);
+
+}  // namespace desktop_ios_promotion
+
+#endif  // CHROME_BROWSER_DESKTOP_PROMOTION_DESKTOP_PROMOTION_PREFS_H