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 e6bcc8094ab5e9a9bd5abed331d49c997cb20e85..84bb58a46b8085ba25fa2ea2002291516a58da89 100644
--- a/components/precache/content/precache_manager.h
+++ b/components/precache/content/precache_manager.h
@@ -70,7 +70,9 @@ class PrecacheManager : public KeyedService,
                   const syncer::SyncService* const sync_service,
                   const history::HistoryService* const history_service,
                   const base::FilePath& db_path,
-                  std::unique_ptr<PrecacheDatabase> precache_database);
+                  std::unique_ptr<PrecacheDatabase> precache_database,
+                  const base::Callback<void(const base::Time&)>&
+                      register_synthetic_field_trial_callback);
   ~PrecacheManager() override;
 
   // Returns true if the client is in the experiment group -- that is,
@@ -142,6 +144,11 @@ 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);
+
   // Callback when fetching unfinished work from storage is done.
   void OnGetUnfinishedWorkDone(
       std::unique_ptr<PrecacheUnfinishedWork> unfinished_work);
@@ -206,6 +213,10 @@ 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&)>
+      register_synthetic_field_trial_callback_;
+
   DISALLOW_COPY_AND_ASSIGN(PrecacheManager);
 };