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

components/sync/user_events/global_id_mapper.h

Index: components/sync/user_events/global_id_mapper.h
diff --git a/components/sync/user_events/global_id_mapper.h b/components/sync/user_events/global_id_mapper.h
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+// 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)>;
+
+// UserEventSpecifics references SESSIONS data through their |navigation_id|
+// which will match values seen in TabNavigation's |global_id|. This field is
+// really just the timestamp from the NavigationEntry, which can change when a
+// page is reloaded. This will cause the SESSIONS side to override the old data.
+// The impact to USER_EVENTS depends on if the previous timestamp made it to the
+// server as part of SESSIONS data or not. If it did not, then USER_EVENTS must
+// be updated. But knowing the answer that question is currently very tricky
+// with Sync's current architecture, so instead we always update for uncommitted
+// USER_EVENTS. The purpose of the GlobalIdMapper is the track changes in the
+// SESSIONS' |global_id| field and feed this information the USER_EVENTS side.
+class GlobalIdMapper {
+ public:
+  // Register for information about changing
+  virtual void AddGlobalIdChangeObserver(GlobalIdChange callback) = 0;
+
+  // Given a |global_id|, returns what the latest 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;
+};
+
+}  // namespace syncer
+
+#endif  // COMPONENTS_SYNC_USER_EVENTS_GLOBAL_ID_MAPPER_H_