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 422260e2e2fc3732879435cc7e4524675284f73a..587ca18a92e9f7fd1608ec0bfd1e7c4f4bebe36a 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[];
 
 extern const char kDismissedRecentOfflineTabSuggestions[];
 extern const char kDismissedDownloadSuggestions[];