[NTP Snippets] Schedule periodic fetching

components/ntp_snippets/ntp_snippets_fetcher.cc

Index: components/ntp_snippets/ntp_snippets_fetcher.cc
diff --git a/components/ntp_snippets/ntp_snippets_fetcher.cc b/components/ntp_snippets/ntp_snippets_fetcher.cc
index f7c82fab41c6cf74a37108c0f03ca1f4dc24909c..00e2e871e67a8c0cd8cbca1250bf70001254b7e4 100644
--- a/components/ntp_snippets/ntp_snippets_fetcher.cc
+++ b/components/ntp_snippets/ntp_snippets_fetcher.cc
@@ -57,6 +57,7 @@ NTPSnippetsFetcher::NTPSnippetsFetcher(
       url_request_context_getter_(url_request_context_getter),
       signin_manager_(signin_manager),
       token_service_(token_service),
+      waiting_for_refresh_token_(false),
       weak_ptr_factory_(this) {}
 
 NTPSnippetsFetcher::~NTPSnippetsFetcher() {}
@@ -89,9 +90,10 @@ void NTPSnippetsFetcher::OnFileExistsCheckDone(bool exists) {
 void NTPSnippetsFetcher::StartFetch() {
   if (signin_manager_->IsAuthenticated()) {
     StartTokenRequest();
-  } else {
+  } else if (!waiting_for_refresh_token_) {
     // Wait until we get a refresh token.
     token_service_->AddObserver(this);
+    waiting_for_refresh_token_ = true;
   }
 }
 
@@ -142,6 +144,7 @@ void NTPSnippetsFetcher::OnGetTokenFailure(
 void NTPSnippetsFetcher::OnRefreshTokenAvailable(
     const std::string& account_id) {
   token_service_->RemoveObserver(this);
+  waiting_for_refresh_token_ = false;
   StartTokenRequest();
 }