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 936b151c44dea06a264957dd0cb6630f2f3a5adb..67e1dc64b30b246b755e1ff51c69b073b0eddaa2 100644
--- a/components/ntp_snippets/pref_names.cc
+++ b/components/ntp_snippets/pref_names.cc
@@ -11,5 +11,10 @@ const char kEnableSnippets[] = "ntp_snippets.enable";
 
 const char kSnippetHosts[] = "ntp_snippets.hosts";
 
+const char kSnippetFetcherQuotaDay[] =
+    "ntp.request_throttler.suggestion_fetcher.day";
+const char kSnippetFetcherQuotaCount[] =
+    "ntp.request_throttler.suggestion_fetcher.count";
+
 }  // namespace prefs
 }  // namespace ntp_snippets