Introduce a request throttler for limiting requests in mobile NTP.

components/ntp_snippets/pref_names.cc

 // 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.
 
 #include "components/ntp_snippets/pref_names.h"
 
 namespace ntp_snippets {
 namespace prefs {
 
 const char kEnableSnippets[] = "ntp_snippets.enable";
 
 const char kSnippetHosts[] = "ntp_snippets.hosts";
 
+const char kSnippetFetcherQuotaDay[] =
+    "ntp.request_throttler.suggestion_fetcher.day";
+const char kSnippetFetcherQuotaCount[] =
+    "ntp.request_throttler.suggestion_fetcher.count";
+
 }  // namespace prefs
 }  // namespace ntp_snippets