Only set precache.last_time when precache ran.

chrome/browser/android/precache/precache_launcher.h

Index: chrome/browser/android/precache/precache_launcher.h
diff --git a/chrome/browser/android/precache/precache_launcher.h b/chrome/browser/android/precache/precache_launcher.h
index 7f80669ff4bb891f725b92b2f53c4f07a4d54082..3fdda2a6150b9d1127fd1e4450a87e5d5306fcf9 100644
--- a/chrome/browser/android/precache/precache_launcher.h
+++ b/chrome/browser/android/precache/precache_launcher.h
@@ -20,8 +20,9 @@ class PrecacheLauncher {
 
  private:
   ~PrecacheLauncher();
-  // Called when precaching completes.
-  void OnPrecacheCompleted();
+  // Called when precaching completes. |precache_started| is true iff the
+  // preconditions were met and the precache started successfully.
+  void OnPrecacheCompleted(bool precache_started);
 
   JavaObjectWeakGlobalRef weak_java_precache_launcher_;