[Sync] Maintain a global_id mapping to update UserEvents that references navigations

components/sync/user_events/global_id_mapper.h

+// Copyright 2017 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 COMPONENTS_SYNC_USER_EVENTS_GLOBAL_ID_MAPPER_H_
+#define COMPONENTS_SYNC_USER_EVENTS_GLOBAL_ID_MAPPER_H_
+
+#include <stdint.h>
+
+#include "base/callback.h"
+
+namespace syncer {
+
+using GlobalIdChange =
+    base::RepeatingCallback<void(int64_t old_global_id, int64_t new_global_id)>;
+
+class GlobalIdMapper {

[Patrick Noland, 2017/06/28 22:44:37]

Please add a comment explaining what this is class is for and why someone would
want to use it. 

[skym, 2017/07/05 19:14:28]

Done.

+ public:
+  // Register for information about changing
+  virtual void AddGlobalIdChangeObserver(GlobalIdChange callback) = 0;
+
+  // Given a |global_id|, returns what the current |global_id| is for the given
+  // navigation, to the best of our ability. If we do not have such a mapping,
+  // which is quite possible, the input |global_id| is returned.
+  virtual int64_t GetLatestGlobalId(int64_t global_id) = 0;

[Patrick Noland, 2017/06/28 22:44:37]

I think I understand what this is doing, but the comment and name are kind of
confusing. First of all I don't think you meant to put | | around the second
global_id. Second, referring to a navigation is confusing since no other code or
comment here refers to navigation at all.  Third, the comment says "current
global id" when the function is named "latest" global id; using consistent
terminology would be helpful. 

[skym, 2017/07/05 19:14:28]

Done the first and third requests, the class comment now includes words about
navigations, so keeping that here as well.

+};
+
+}  // namespace syncer
+
+#endif  // COMPONENTS_SYNC_USER_EVENTS_GLOBAL_ID_MAPPER_H_