Create a synthetic field trial for precache.

components/precache/content/precache_manager.h

Index: components/precache/content/precache_manager.h
diff --git a/components/precache/content/precache_manager.h b/components/precache/content/precache_manager.h
index ec093b0057a520aa177ac8fd282f8d7cae5fc70d..eee04410b416df18cb5c72f65f28b4fc67b255ed 100644
--- a/components/precache/content/precache_manager.h
+++ b/components/precache/content/precache_manager.h
@@ -81,7 +81,9 @@ class PrecacheManager : public KeyedService,
                   const data_reduction_proxy::DataReductionProxySettings*
                       data_reduction_proxy_settings,
                   const base::FilePath& db_path,
-                  std::unique_ptr<PrecacheDatabase> precache_database);
+                  std::unique_ptr<PrecacheDatabase> precache_database,
+                  const base::Callback<void(const base::Time&)>&

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

Idem.

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

Done.

+                      register_synthetic_field_trial_callback);
   ~PrecacheManager() override;
 
   // Returns true if the client is in the experiment group -- that is,
@@ -153,6 +155,11 @@ class PrecacheManager : public KeyedService,
   // From PrecacheFetcher::PrecacheDelegate.
   void OnDone() override;
 
+  // Registers the precache synthetic field trial for users whom the precache
+  // task was run recently. |last_precache_time| is the last time precache task
+  // was run.
+  void RegisterSyntheticFieldTrial(const base::Time& last_precache_time);

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

Idem.

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

Done.

+
   // Callback when fetching unfinished work from storage is done.
   void OnGetUnfinishedWorkDone(
       std::unique_ptr<PrecacheUnfinishedWork> unfinished_work);
@@ -239,6 +246,10 @@ class PrecacheManager : public KeyedService,
   // Work that hasn't yet finished.
   std::unique_ptr<PrecacheUnfinishedWork> unfinished_work_;
 
+  // Callback to register the synthetic field trial.
+  const base::Callback<void(const base::Time&)>

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

Idem.

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

Done.

+      register_synthetic_field_trial_callback_;
+
   DISALLOW_COPY_AND_ASSIGN(PrecacheManager);
 };