[Sync] Add more info for invalid position check

sync/syncable/directory.cc

 // Copyright 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.
 
 #include "sync/syncable/directory.h"
 
 #include <algorithm>
 #include <iterator>
 
 #include "base/base64.h"
@@ skipping 1451 matching lines @@
     return;
   }
 
   EntryKernel* successor = *neighbour;
 
   // Another mixed valid and invalid position case.  This one could be supported
   // in theory, but we're trying to deprecate support for siblings with and
   // 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(
       predecessor->ref(UNIQUE_POSITION),
       successor->ref(UNIQUE_POSITION),
       suffix);
   e->put(UNIQUE_POSITION, pos);
   return;
 }
 
@@ skipping 79 matching lines @@
 Directory::Kernel* Directory::kernel() {
   return kernel_;
 }
 
 const Directory::Kernel* Directory::kernel() const {
   return kernel_;
 }
 
 }  // namespace syncable
 }  // namespace syncer