[Sync] Fix a crash when sign into sync.

chrome/browser/android/most_visited_sites.cc

 // Copyright 2013 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.
 
 #include "chrome/browser/android/most_visited_sites.h"
 
 #include <string>
 #include <vector>
 
 #include "base/android/jni_android.h"
@@ skipping 191 matching lines @@
   // Register this class as an observer to the sync service. It is important to
   // be notified of changes in the sync state such as initialization, sync
   // being enabled or disabled, etc.
   ProfileSyncService* profile_sync_service =
       ProfileSyncServiceFactory::GetForProfile(profile_);
   if (profile_sync_service)
     profile_sync_service->AddObserver(this);
 }
 
 MostVisitedSites::~MostVisitedSites() {
+  ProfileSyncService* profile_sync_service =
+      ProfileSyncServiceFactory::GetForProfile(profile_);
+  if (profile_sync_service && profile_sync_service->HasObserver(this))
+    profile_sync_service->RemoveObserver(this);
 }
 
 void MostVisitedSites::Destroy(JNIEnv* env, jobject obj) {
   delete this;
 }
 
 void MostVisitedSites::OnLoadingComplete(JNIEnv* env, jobject obj) {
   RecordUMAMetrics();
 }
 
@@ skipping 290 matching lines @@
   num_empty_thumbs_ = 0;
   UMA_HISTOGRAM_SPARSE_SLOWLY(kNumServerTilesHistogramName, num_server_thumbs_);
   num_server_thumbs_ = 0;
 }
 
 static jlong Init(JNIEnv* env, jobject obj, jobject jprofile) {
   MostVisitedSites* most_visited_sites =
       new MostVisitedSites(ProfileAndroid::FromProfileAndroid(jprofile));
   return reinterpret_cast<intptr_t>(most_visited_sites);
 }