Only set precache.last_time when precache ran.

chrome/browser/android/precache/precache_launcher.h

 // Copyright 2014 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_ANDROID_PRECACHE_PRECACHE_LAUNCHER_H_
 #define CHROME_BROWSER_ANDROID_PRECACHE_PRECACHE_LAUNCHER_H_
 
 #include <jni.h>
 
 #include "base/android/jni_android.h"
 #include "base/android/jni_weak_ref.h"
 #include "base/memory/weak_ptr.h"
 
 class PrecacheLauncher {
  public:
   PrecacheLauncher(JNIEnv* env, jobject obj);
   void Destroy(JNIEnv* env, jobject obj);
   void Start(JNIEnv* env, jobject obj);
   void Cancel(JNIEnv* env, jobject obj);
 
  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_;
 
   // This must be the last member field in the class.
   base::WeakPtrFactory<PrecacheLauncher> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(PrecacheLauncher);
 };
 
 // Registers the native methods to be called from Java.
 bool RegisterPrecacheLauncher(JNIEnv* env);
 
 #endif  // CHROME_BROWSER_ANDROID_PRECACHE_PRECACHE_LAUNCHER_H_