Add request throttler to thumbnail fetching for articles on mobile NTP

components/ntp_snippets/pref_names.h

 // 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.
 
 #ifndef COMPONENTS_NTP_SNIPPETS_PREF_NAMES_H_
 #define COMPONENTS_NTP_SNIPPETS_PREF_NAMES_H_
 
 namespace ntp_snippets {
 namespace prefs {
 
 extern const char kEnableSnippets[];
 
 extern const char kSnippetHosts[];
 
 // The pref name for today's count of NTPSnippetsFetcher requests, so far.
-extern const char kSnippetFetcherQuotaCount[];
+extern const char kSnippetFetcherRequestCount[];
+// The pref name for today's count of NTPSnippetsFetcher interactive requests.
+extern const char kSnippetFetcherInteractiveRequestCount[];
 // The pref name for the current day for the counter of NTPSnippetsFetcher
 // requests.
-extern const char kSnippetFetcherQuotaDay[];
+extern const char kSnippetFetcherRequestsDay[];
+
+// The pref name for today's count of requests for article thumbnails, so far.
+extern const char kSnippetThumbnailsRequestCount[];
+// The pref name for today's count of interactive requests for article
+// thumbnails, so far.
+extern const char kSnippetThumbnailsInteractiveRequestCount[];
+// The pref name for the current day for the counter of requests for article
+// thumbnails.
+extern const char kSnippetThumbnailsRequestsDay[];
 
 }  // namespace prefs
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_PREF_NAMES_H_