[Sync] Add more info for invalid position check

sync/syncable/directory.cc

Index: sync/syncable/directory.cc
diff --git a/sync/syncable/directory.cc b/sync/syncable/directory.cc
index 720a924e4a6b801a802e55ad825834e7e2509542..1a5faf1f876c4852c53a9ea46a5cd4aef1dd0101 100644
--- a/sync/syncable/directory.cc
+++ b/sync/syncable/directory.cc
@@ -1469,7 +1469,7 @@ void Directory::PutPredecessor(EntryKernel* e, EntryKernel* predecessor) {
   // without valid positions.  See TODO above.
   // Using a release CHECK here because the following UniquePosition::Between
   // call crashes anyway when the position string is empty (see crbug/332371).
-  CHECK(successor->ref(UNIQUE_POSITION).IsValid());
+  CHECK(successor->ref(UNIQUE_POSITION).IsValid()) << *successor;
 
   // Finally, the normal case: inserting between two elements.
   UniquePosition pos = UniquePosition::Between(