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

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

Index: chrome/browser/extensions/api/storage/sync_value_store_cache.cc
diff --git a/chrome/browser/extensions/api/storage/sync_value_store_cache.cc b/chrome/browser/extensions/api/storage/sync_value_store_cache.cc
index 0a440ee84f6c218b8488b00df6c553a05f5219f7..33267b80f23e5c18669e58bb3a8c0db2dadd4b1a 100644
--- a/chrome/browser/extensions/api/storage/sync_value_store_cache.cc
+++ b/chrome/browser/extensions/api/storage/sync_value_store_cache.cc
@@ -35,7 +35,7 @@ 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
@@ -52,6 +52,10 @@ 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);