[ios] Adds an Initialize() method to OmahaServiceProvider.

ios/public/provider/chrome/browser/omaha/omaha_service_provider.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/public/provider/chrome/browser/omaha/omaha_service_provider.h"
 
 OmahaServiceProvider::OmahaServiceProvider() {}
 
 OmahaServiceProvider::~OmahaServiceProvider() {}
 
+void OmahaServiceProvider::Initialize() {}
+
 GURL OmahaServiceProvider::GetUpdateServerURL() const {
   return GURL();
 }
 
 std::string OmahaServiceProvider::GetApplicationID() const {
   return std::string();
 }
 
 std::string OmahaServiceProvider::GetBrandCode() const {
   return std::string();
 }
 
 void OmahaServiceProvider::AppendExtraAttributes(const std::string& tag,
                                                  OmahaXmlWriter* writer) const {
 }