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 24130b9944627d344503fd1a595a311cba0364e0..b9292d38a812c9206ca7e231c9348d1813b0b132 100644
--- a/components/ntp_snippets/pref_names.h
+++ b/components/ntp_snippets/pref_names.h
@@ -12,7 +12,12 @@ extern const char kDisableSnippets[];
 
 extern const char kSnippetHosts[];
 
+// Current day for counting NTPSnippetsFetcher requests.
+extern const char kSnippetFetcherQuotaDay[];
+// The count of NTPSnippetsFetcher requests for today, so far.
+extern const char kSnippetFetcherQuotaCount[];
+
 }  // namespace prefs
 }  // namespace ntp_snippets
 
-#endif
+#endif  // COMPONENTS_NTP_SNIPPETS_PREF_NAMES_H_