Only set precache.last_time when precache ran.

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 912a41fc055467bd7689ea344427c34f2ff79150..166379a6c4650a55774eb04100f0be8b59cbd2eb 100644
--- a/components/precache/content/precache_manager.h
+++ b/components/precache/content/precache_manager.h
@@ -54,7 +54,7 @@ class PrecacheManager : public KeyedService,
                         public PrecacheFetcher::PrecacheDelegate,
                         public base::SupportsWeakPtr<PrecacheManager> {
  public:
-  typedef base::Closure PrecacheCompletionCallback;
+  typedef base::Callback<void(bool)> PrecacheCompletionCallback;
 
   PrecacheManager(content::BrowserContext* browser_context,
                   const sync_driver::SyncService* const sync_service);
@@ -71,8 +71,9 @@ class PrecacheManager : public KeyedService,
 
   // Starts precaching resources that the user is predicted to fetch in the
   // future. If precaching is already currently in progress, then this method
-  // does nothing. The |precache_completion_callback| will be run when
-  // precaching finishes, but will not be run if precaching is canceled.
+  // does nothing. The |precache_completion_callback| will be passed true when
+  // precaching finishes, and passed false when precaching abort due to failed
+  // preconditions, but will not be run if precaching is canceled.
   void StartPrecaching(
       const PrecacheCompletionCallback& precache_completion_callback,
       const history::HistoryService& history_service);