[NTP Snippets] Add histogram with fetch time

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>
 #include <vector>
 
 #include "base/callback.h"
 #include "base/memory/weak_ptr.h"
 #include "base/optional.h"
+#include "base/time/tick_clock.h"
+#include "base/time/time.h"
 #include "components/ntp_snippets/ntp_snippet.h"
 #include "net/url_request/url_fetcher_delegate.h"
 #include "net/url_request/url_request_context_getter.h"
 
 namespace base {
 class Value;
 }
 
 namespace ntp_snippets {
 
@@ skipping 45 matching lines @@
   void FetchSnippetsFromHosts(const std::set<std::string>& hosts, int count);
 
   // Debug string representing the status/result of the last fetch attempt.
   const std::string& last_status() const { return last_status_; }
 
   // Returns the last json fetched from the server.
   const std::string& last_json() const {
     return last_fetch_json_;
   }
 
+  // Overrides internal clock for testing purposes.
+  void SetTickClockForTesting(std::unique_ptr<base::TickClock> tick_clock) {
+    tick_clock_ = std::move(tick_clock);
+  }
+
  private:
   // URLFetcherDelegate implementation.
   void OnURLFetchComplete(const net::URLFetcher* source) override;
 
   void OnJsonParsed(std::unique_ptr<base::Value> parsed);
   void OnJsonError(const std::string& error);
   void FetchFinished(OptionalSnippets snippets,
                      FetchResult result,
                      const std::string& extra_message);
 
   // Holds the URL request context.
   scoped_refptr<net::URLRequestContextGetter> url_request_context_getter_;
 
   const ParseJSONCallback parse_json_callback_;
+  base::TimeTicks fetch_start_time_;
   std::string last_status_;
   std::string last_fetch_json_;
 
   // The fetcher for downloading the snippets.
   std::unique_ptr<net::URLFetcher> url_fetcher_;
 
   // The callback to notify when new snippets get fetched.
   SnippetsAvailableCallback snippets_available_callback_;
 
   // Flag for picking the right (stable/non-stable) API key for Chrome Reader
   bool is_stable_channel_;
 
+  // Allow for an injectable tick clock for testing.
+  std::unique_ptr<base::TickClock> tick_clock_;
+
   base::WeakPtrFactory<NTPSnippetsFetcher> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(NTPSnippetsFetcher);
 };
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_NTP_SNIPPETS_FETCHER_H_