[SyncFS] Add last validate time in MDDB ver.4

chrome/browser/sync_file_system/drive_backend/drive_backend_constants.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 "chrome/browser/sync_file_system/drive_backend/drive_backend_constants.h"
 
 namespace sync_file_system {
 namespace drive_backend {
 
 const char kSyncRootFolderTitle[] = "Chrome Syncable FileSystem";
 const char kSyncRootFolderTitleDev[] = "Chrome Syncable FileSystem Dev";
 const char kMimeTypeOctetStream[] = "application/octet-stream";
 
 const base::FilePath::CharType kDatabaseName[] =
     FILE_PATH_LITERAL("DriveMetadata_v2");
 
 const char kDatabaseVersionKey[] = "VERSION";
 const int64 kCurrentDatabaseVersion = 3;
 const int64 kDatabaseOnDiskVersion = 4;
 const char kServiceMetadataKey[] = "SERVICE";
 const char kFileMetadataKeyPrefix[] = "FILE: ";
 const char kFileTrackerKeyPrefix[] = "TRACKER: ";
+const char kLastValidationTimeKey[] = "LAST_VALID";
 
 const char kAppRootIDByAppIDKeyPrefix[] = "APP_ROOT: ";
 const char kActiveTrackerIDByFileIDKeyPrefix[] = "ACTIVE_FILE: ";
 const char kTrackerIDByFileIDKeyPrefix[] = "TRACKER_FILE: ";
 const char kMultiTrackerByFileIDKeyPrefix[] = "MULTI_FILE: ";
 const char kActiveTrackerIDByParentAndTitleKeyPrefix[] = "ACTIVE_PATH: ";
 const char kTrackerIDByParentAndTitleKeyPrefix[] = "TRACKER_PATH: ";
 const char kMultiBackingParentAndTitleKeyPrefix[] = "MULTI_PATH: ";
 const char kDirtyIDKeyPrefix[] = "DIRTY: ";
 const char kDemotedDirtyIDKeyPrefix[] = "DEMOTED_DIRTY: ";
 
 const int kMaxRetry = 5;
 const int64 kListChangesRetryDelaySeconds = 60 * 60;
 
 const int64 kInvalidTrackerID = 0;
 
 }  // namespace drive_backend
 }  // namespace sync_file_system