sync: Use a single transaction in ProcessUpdates rather than one per entry.

chrome/browser/sync/engine/process_updates_command.h

Index: chrome/browser/sync/engine/process_updates_command.h
diff --git a/chrome/browser/sync/engine/process_updates_command.h b/chrome/browser/sync/engine/process_updates_command.h
index f9ff968bf4293354511c3d3df569dea90ef32364..bf34b6123926880284a4f7edab7bc241fd10fcb6 100644
--- a/chrome/browser/sync/engine/process_updates_command.h
+++ b/chrome/browser/sync/engine/process_updates_command.h
@@ -11,6 +11,7 @@
 
 namespace syncable {
 class ScopedDirLookup;
+class WriteTransaction;
 }
 
 namespace sync_pb {
@@ -39,7 +40,8 @@ class ProcessUpdatesCommand : public ModelChangingSyncerCommand {
  private:
   ServerUpdateProcessingResult ProcessUpdate(
       const syncable::ScopedDirLookup& dir,
-      const sync_pb::SyncEntity& proto_update);
+      const sync_pb::SyncEntity& proto_update,
+      syncable::WriteTransaction* const trans);
   DISALLOW_COPY_AND_ASSIGN(ProcessUpdatesCommand);
 };