Create a synthetic field trial for precache.

chrome/browser/precache/precache_util.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 CHROME_BROWSER_PRECACHE_PRECACHE_UTIL_H_
 #define CHROME_BROWSER_PRECACHE_PRECACHE_UTIL_H_
 
+#include "base/time/time.h"
+
 namespace net {
 class URLRequest;
 }
 
 namespace precache {
 
 void UpdatePrecacheMetricsAndState(const net::URLRequest* request,
                                    void* profile_id);
 
+// Registers the precache synthetic field trial for users whom the precache task
+// started in the last |kDaysForPrecacheCandidates| days. The group name of the
+// synthetic field trial will be the regular precache group.
+// |last_precache_time| is the last time precache task was run.
+void RegisterPrecacheSyntheticFieldTrial(const base::Time& last_precache_time);

[jamartin, 2016/12/21 23:54:33]

// All time classes are copyable, assignable, and occupy 64-bits per instance.
// As a result, prefer passing them by value:

[twifkak, 2017/01/05 23:53:14]

Done.

+
 }  // namespace precache
 
 #endif  // CHROME_BROWSER_PRECACHE_PRECACHE_UTIL_H_