[SyncFS] Implement MetadataDatabase initialization

chrome/browser/sync_file_system/drive_backend/metadata_database.proto

 // 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.
 //
 // Protocol buffer definitions for Drive backend of Syncable FileSystem.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
 package sync_file_system.drive_backend;
 
 enum FileKind {
   KIND_UNSUPPORTED = 0;
   KIND_FILE = 1;
   KIND_FOLDER = 2;
 }
 
 message ServiceMetadata {
-  optional int64 largest_change_id = 1 [default = 0];
+  optional int64 largest_change_id = 1;
   optional string sync_root_folder_id = 2;
 }
 
 message DriveFileMetadata {
   // File ID of the remote file/folder which the DriveFileMetadata tracks.
   required string file_id = 1;
   required string parent_folder_id = 2;
 
   optional string app_id = 3;
   optional bool is_app_root = 4;
@@ skipping 31 matching lines @@
   // True if the DriveFileMetadata is active.
   // Remote file content update should only be applied for active
   // DriveFileMetadata.
   // Active DriveFileMetadata must have a unique title under its parent.
   optional bool active = 8;
 
   // Valid only for folders.
   // True indicates the folder contents has not yet been fetched.
   optional bool needs_folder_listing = 9;
 }