Add request throttler to thumbnail fetching for articles on mobile NTP

components/ntp_snippets/pref_names.h

Index: components/ntp_snippets/pref_names.h
diff --git a/components/ntp_snippets/pref_names.h b/components/ntp_snippets/pref_names.h
index 4fdea4ebba7065ad0935aa66825b9bf055e1dc6a..3c594a4e7e68ea85d813ca7adf07180681fba5bc 100644
--- a/components/ntp_snippets/pref_names.h
+++ b/components/ntp_snippets/pref_names.h
@@ -13,10 +13,21 @@ 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