[sql] Consider fresh databases suitable for memory-mapped I/O.

sync/syncable/directory_backing_store.cc

Index: sync/syncable/directory_backing_store.cc
diff --git a/sync/syncable/directory_backing_store.cc b/sync/syncable/directory_backing_store.cc
index 55c99d38264ac5be42a54fbfd7dcf369da6fb74e..fa1b94770e4e6db466e30d4889be015a6605b03e 100644
--- a/sync/syncable/directory_backing_store.cc
+++ b/sync/syncable/directory_backing_store.cc
@@ -1728,9 +1728,10 @@ void DirectoryBackingStore::ResetAndCreateConnection() {
   db_->set_cache_size(32);
   db_->set_page_size(database_page_size_);
 
-  // TODO(shess): Sync corruption tests interact poorly with mmap, disable for
-  // now.  http://crbug.com/533682
+  // TODO(shess): The current mitigation for http://crbug.com/537742 stores
+  // state in the meta table, which this database does not use.
   db_->set_mmap_disabled();
+
   if (!catastrophic_error_handler_.is_null())
     SetCatastrophicErrorHandler(catastrophic_error_handler_);
 }