sync: Test and improve bookmark performance

sync/syncable/syncable_write_transaction.cc

Index: sync/syncable/syncable_write_transaction.cc
diff --git a/sync/syncable/syncable_write_transaction.cc b/sync/syncable/syncable_write_transaction.cc
index 5b884e7f3581a99935153c8b923ac0c7514d8b9b..478792634091a3ed52fc188fcc8cfb43714cdce2 100644
--- a/sync/syncable/syncable_write_transaction.cc
+++ b/sync/syncable/syncable_write_transaction.cc
@@ -40,9 +40,7 @@ void WriteTransaction::SaveOriginal(const EntryKernel* entry) {
   const int64 handle = entry->ref(META_HANDLE);
   EntryKernelMutationMap::iterator it = mutations_.lower_bound(handle);
   if (it == mutations_.end() || it->first != handle) {
-    EntryKernelMutation mutation;
-    mutation.original = *entry;
-    ignore_result(mutations_.insert(it, std::make_pair(handle, mutation)));
+    mutations_[handle].original = *entry;
   }
 }