[SyncFS] Add UMAs to track syncfs usage

chrome/browser/sync_file_system/conflict_resolution_policy.h

 // Copyright (c) 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.
 
 #ifndef CHROME_BROWSER_SYNC_FILE_SYSTEM_CONFLICT_RESOLUTION_POLICY_H_
 #define CHROME_BROWSER_SYNC_FILE_SYSTEM_CONFLICT_RESOLUTION_POLICY_H_
 
 namespace sync_file_system {
 
 enum ConflictResolutionPolicy {
   // Resolution policy unknown or not initialized. Usually indicates an error.
-  CONFLICT_RESOLUTION_POLICY_UNKNOWN,
+  CONFLICT_RESOLUTION_POLICY_UNKNOWN = 0,
 
   // The service automatically resolves a conflict by choosing the one
   // that is updated more recently.
   CONFLICT_RESOLUTION_POLICY_LAST_WRITE_WIN,
 
   // The service does nothing and just leaves conflicting files in
   // 'conflicted' state.
   CONFLICT_RESOLUTION_POLICY_MANUAL,
+
+  CONFLICT_RESOLUTION_POLICY_MAX,
 };
 
 }  // namespace sync_file_system
 
 #endif  // CHROME_BROWSER_SYNC_FILE_SYSTEM_CONFLICT_RESOLUTION_POLICY_H_