Introduce a request throttler for limiting requests in 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 5c432d6f52db03445f32db5c5e3aa0545a540c56..4fdea4ebba7065ad0935aa66825b9bf055e1dc6a 100644
--- a/components/ntp_snippets/pref_names.h
+++ b/components/ntp_snippets/pref_names.h
@@ -12,7 +12,13 @@ extern const char kEnableSnippets[];
 
 extern const char kSnippetHosts[];
 
+// The pref name for today's count of NTPSnippetsFetcher requests, so far.
+extern const char kSnippetFetcherQuotaCount[];
+// The pref name for the current day for the counter of NTPSnippetsFetcher
+// requests.
+extern const char kSnippetFetcherQuotaDay[];
+
 }  // namespace prefs
 }  // namespace ntp_snippets
 
-#endif
+#endif  // COMPONENTS_NTP_SNIPPETS_PREF_NAMES_H_