sync: Attempt to recover from directory corruption

sync/syncable/directory_backing_store.h

Index: sync/syncable/directory_backing_store.h
diff --git a/sync/syncable/directory_backing_store.h b/sync/syncable/directory_backing_store.h
index 75fdad175da2685500c3bddf7e335f5c90a9ef93..ed998e28467b0eefba7b035c2e37dcc61c489c31 100644
--- a/sync/syncable/directory_backing_store.h
+++ b/sync/syncable/directory_backing_store.h
@@ -126,6 +126,9 @@ class DirectoryBackingStore : public base::NonThreadSafe {
   // with an error message.
   bool CheckIntegrity(sqlite3* handle, std::string* error) const;
 
+  // Checks that the references between sync nodes is consistent.
+  static bool VerifyReferenceIntegrity(const MetahandlesIndex& entries);
+
   // Migration utilities.
   bool RefreshColumns();
   bool SetVersion(int version);