[NTP Snippets] Re-fetch snippets on ML changes only when the hosts actually change

components/ntp_snippets/ntp_snippets_fetcher.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/ntp_snippets/ntp_snippets_fetcher.h"
 
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/path_service.h"
 #include "base/strings/string_util.h"
@@ skipping 48 matching lines @@
       weak_ptr_factory_(this) {}
 
 NTPSnippetsFetcher::~NTPSnippetsFetcher() {
 }
 
 scoped_ptr<NTPSnippetsFetcher::SnippetsAvailableCallbackList::Subscription>
 NTPSnippetsFetcher::AddCallback(const SnippetsAvailableCallback& callback) {
   return callback_list_.Add(callback);
 }
 
-void NTPSnippetsFetcher::FetchSnippets(const std::vector<std::string>& hosts) {
+void NTPSnippetsFetcher::FetchSnippets(const std::set<std::string>& hosts) {
   // TODO(treib): What to do if there's already a pending request?
   const std::string& key = is_stable_channel_
                                ? google_apis::GetAPIKey()
                                : google_apis::GetNonStableAPIKey();
   std::string url =
       base::StringPrintf(kContentSnippetsServerFormat, key.c_str());
   url_fetcher_ = URLFetcher::Create(GURL(url), URLFetcher::POST, this);
   url_fetcher_->SetRequestContext(url_request_context_getter_.get());
   url_fetcher_->SetLoadFlags(net::LOAD_DO_NOT_SEND_COOKIES |
                              net::LOAD_DO_NOT_SAVE_COOKIES);
@@ skipping 34 matching lines @@
   }
 
   std::string response;
   bool stores_result_to_string = source->GetResponseAsString(&response);
   DCHECK(stores_result_to_string);
 
   callback_list_.Notify(response);
 }
 
 }  // namespace ntp_snippets