[NTP Snippets] Add unit tests for NTPSnippetsFetcher

components/ntp_snippets/ntp_snippets_fetcher.h

 // 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.
 
 #ifndef COMPONENTS_NTP_SNIPPETS_NTP_SNIPPETS_FETCHER_H_
 #define COMPONENTS_NTP_SNIPPETS_NTP_SNIPPETS_FETCHER_H_
 
 #include <memory>
 #include <set>
 #include <string>
@@ skipping 24 matching lines @@
       bool is_stable_channel);
   ~NTPSnippetsFetcher() override;
 
   // Adds a callback that is called when a new set of snippets are downloaded.
   std::unique_ptr<SnippetsAvailableCallbackList::Subscription> AddCallback(
       const SnippetsAvailableCallback& callback) WARN_UNUSED_RESULT;
 
   // Fetches snippets from the server. |hosts| can be used to restrict the
   // results to a set of hosts, e.g. "www.google.com". If it is empty, no
   // restrictions are applied.
+  //
+  // If an ongoing fetch exists, it will be cancelled and a new one started,
+  // without triggering additional callbacks (i.e. not noticeable by
+  // subscribers).
   void FetchSnippets(const std::set<std::string>& hosts, int count);
 
  private:
   // URLFetcherDelegate implementation.
   void OnURLFetchComplete(const net::URLFetcher* source) override;
 
   // The SequencedTaskRunner on which file system operations will be run.
   scoped_refptr<base::SequencedTaskRunner> file_task_runner_;
 
   // Holds the URL request context.
   scoped_refptr<net::URLRequestContextGetter> url_request_context_getter_;
 
   // The fetcher for downloading the snippets.
   std::unique_ptr<net::URLFetcher> url_fetcher_;
 
   // The callbacks to notify when new snippets get fetched.
   SnippetsAvailableCallbackList callback_list_;
 
   // Flag for picking the right (stable/non-stable) API key for Chrome Reader
   bool is_stable_channel_;
 
   DISALLOW_COPY_AND_ASSIGN(NTPSnippetsFetcher);
 };
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_NTP_SNIPPETS_FETCHER_H_