[Sync] Stop accessing BrowserContextKeyedServiceFactory on non-UI thread.

chrome/browser/extensions/api/storage/sync_value_store_cache.cc

 // 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.
 
 #include "chrome/browser/extensions/api/storage/sync_value_store_cache.h"
 
 #include <stddef.h>
 
 #include "chrome/browser/extensions/api/storage/sync_storage_backend.h"
 #include "chrome/browser/sync/glue/sync_start_util.h"
@@ skipping 17 matching lines @@
       static_cast<size_t>(api::storage::sync::MAX_ITEMS)};
   return limits;
 }
 
 }  // namespace
 
 SyncValueStoreCache::SyncValueStoreCache(
     const scoped_refptr<ValueStoreFactory>& factory,
     const scoped_refptr<SettingsObserverList>& observers,
     const base::FilePath& profile_path)
-    : initialized_(false) {
+    : initialized_(false), weak_ptr_factory_(this) {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
 
   // This post is safe since the destructor can only be invoked from the
   // same message loop, and any potential post of a deletion task must come
   // after the constructor returns.
   BrowserThread::PostTask(
       BrowserThread::FILE, FROM_HERE,
       base::Bind(&SyncValueStoreCache::InitOnFileThread,
                  base::Unretained(this),
                  factory, observers, profile_path));
 }
 
 SyncValueStoreCache::~SyncValueStoreCache() {
   DCHECK_CURRENTLY_ON(BrowserThread::FILE);
 }
 
+base::WeakPtr<SyncValueStoreCache> SyncValueStoreCache::AsWeakPtr() {
+  return weak_ptr_factory_.GetWeakPtr();
+}
+
 syncer::SyncableService* SyncValueStoreCache::GetSyncableService(
     syncer::ModelType type) const {
   DCHECK_CURRENTLY_ON(BrowserThread::FILE);
   DCHECK(initialized_);
 
   switch (type) {
     case syncer::APP_SETTINGS:
       return app_backend_.get();
     case syncer::EXTENSION_SETTINGS:
       return extension_backend_.get();
@@ skipping 34 matching lines @@
   extension_backend_.reset(new SyncStorageBackend(
       factory,
       GetSyncQuotaLimits(),
       observers,
       syncer::EXTENSION_SETTINGS,
       sync_start_util::GetFlareForSyncableService(profile_path)));
   initialized_ = true;
 }
 
 }  // namespace extensions