[Sync] Fix double tranaction acquire in favicon code

chrome/browser/sync/glue/favicon_cache.h

 // Copyright (c) 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.
 
 #ifndef CHROME_BROWSER_SYNC_GLUE_FAVICON_CACHE_H_
 #define CHROME_BROWSER_SYNC_GLUE_FAVICON_CACHE_H_
 
 #include <map>
 #include <string>
 
@@ skipping 82 matching lines @@
   // synced.
   void OnPageFaviconUpdated(const GURL& page_url);
 
   // Update the visit count for the favicon associated with |favicon_url|.
   // If no favicon exists associated with |favicon_url|, triggers a load
   // for the favicon associated with |page_url|.
   void OnFaviconVisited(const GURL& page_url, const GURL& favicon_url);
 
   // Consume Session sync favicon data. Will not overwrite existing favicons.
   // If |icon_bytes| is empty, only updates the page->favicon url mapping.
+  // Safe to call within a transaction.
   void OnReceivedSyncFavicon(const GURL& page_url,
                              const GURL& icon_url,
                              const std::string& icon_bytes,
                              int64 visit_time_ms);
 
   // Support for syncing favicons using the legacy format (within tab sync).
   void SetLegacyDelegate(FaviconCacheObserver* observer);
   void RemoveLegacyDelegate();
 
   // NotificationObserver implementation.
@@ skipping 129 matching lines @@
 
   // Maximum number of favicons to sync. 0 means no limit.
   const size_t max_sync_favicon_limit_;
 
   DISALLOW_COPY_AND_ASSIGN(FaviconCache);
 };
 
 }  // namespace browser_sync
 
 #endif  // CHROME_BROWSER_SYNC_GLUE_FAVICON_CACHE_H_