Introduce a request throttler for limiting requests in mobile NTP.

components/ntp_snippets/pref_names.cc

Index: components/ntp_snippets/pref_names.cc
diff --git a/components/ntp_snippets/pref_names.cc b/components/ntp_snippets/pref_names.cc
index 43888038db8429396abd8688913d5475dab8d2e0..671a780ef748b3060032f3658c777eb37e2a712b 100644
--- a/components/ntp_snippets/pref_names.cc
+++ b/components/ntp_snippets/pref_names.cc
@@ -11,5 +11,8 @@ const char kDisableSnippets[] = "ntp_snippets.disable";
 
 const char kSnippetHosts[] = "ntp_snippets.hosts";
 
+const char kSnippetFetcherQuotaDay[] = "ntp_snippets.quota.fetcher.day";
+const char kSnippetFetcherQuotaCount[] = "ntp_snippets.quota.fetcher.count";
+
 }  // namespace prefs
 }  // namespace ntp_snippets