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

sync/syncable/directory_backing_store.cc

 // Copyright 2012 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_backing_store.h"
 
 #include <limits>
 
 #include "base/base64.h"
 #include "base/logging.h"
@@ skipping 1710 matching lines @@
   return needs_metas_column_refresh_ || needs_share_info_column_refresh_;
 }
 
 void DirectoryBackingStore::ResetAndCreateConnection() {
   db_.reset(new sql::Connection());
   db_->set_histogram_tag("SyncDirectory");
   db_->set_exclusive_locking();
   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_);
 }
 
 void DirectoryBackingStore::SetCatastrophicErrorHandler(
     const base::Closure& catastrophic_error_handler) {
   DCHECK(CalledOnValidThread());
   DCHECK(!catastrophic_error_handler.is_null());
   catastrophic_error_handler_ = catastrophic_error_handler;
   sql::Connection::ErrorCallback error_callback =
       base::Bind(&OnSqliteError, catastrophic_error_handler_);
   db_->set_error_callback(error_callback);
 }
 
 }  // namespace syncable
 }  // namespace syncer