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

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

Index: chrome/browser/extensions/api/storage/sync_value_store_cache.h
diff --git a/chrome/browser/extensions/api/storage/sync_value_store_cache.h b/chrome/browser/extensions/api/storage/sync_value_store_cache.h
index 6280582386573165fbd1ce260114b731ae323104..2984522013439ca01368df32fada12c3c93847e9 100644
--- a/chrome/browser/extensions/api/storage/sync_value_store_cache.h
+++ b/chrome/browser/extensions/api/storage/sync_value_store_cache.h
@@ -6,10 +6,12 @@
 #define CHROME_BROWSER_EXTENSIONS_API_STORAGE_SYNC_VALUE_STORE_CACHE_H_
 
 #include <memory>
+#include <string>
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
+#include "base/memory/weak_ptr.h"
 #include "components/sync/model/syncable_service.h"
 #include "extensions/browser/api/storage/settings_observer.h"
 #include "extensions/browser/api/storage/value_store_cache.h"
@@ -36,6 +38,8 @@ class SyncValueStoreCache : public ValueStoreCache {
                       const base::FilePath& profile_path);
   ~SyncValueStoreCache() override;
 
+  base::WeakPtr<SyncValueStoreCache> AsWeakPtr();
+
   syncer::SyncableService* GetSyncableService(syncer::ModelType type) const;
 
   // ValueStoreCache implementation:
@@ -52,6 +56,7 @@ class SyncValueStoreCache : public ValueStoreCache {
   bool initialized_;
   std::unique_ptr<SyncStorageBackend> app_backend_;
   std::unique_ptr<SyncStorageBackend> extension_backend_;
+  base::WeakPtrFactory<SyncValueStoreCache> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(SyncValueStoreCache);
 };