[Sync] Creating sync_sessions component, moving revisit logic.

components/sync_driver/revisit/offset_tab_matcher.h

-// Copyright 2015 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_DRIVER_OFFSET_TAB_MATCHER_H_
-#define COMPONENTS_SYNC_DRIVER_OFFSET_TAB_MATCHER_H_
-
-#include "components/sessions/core/session_types.h"
-#include "components/sync_driver/revisit/page_equality.h"
-#include "components/sync_driver/revisit/page_visit_observer.h"
-
-namespace sync_driver {
-
-// This class looks for tabs that have matching pages that are not the current
-// navigation entry. This corresponds to the pages you would arrive at if you
-// pressed the forward/backwards buttons. The goal is to emit metrics for the
-// most recent/current entry. So to break ties of multiple entries that match,
-// first the timestamp on the tab is used, followed by the index of the entry.
-// This isn't necessarily perfect at determining the most recent, but it should
-// be a reasonable enough approximation.
-class OffsetTabMatcher {
- public:
-  explicit OffsetTabMatcher(const PageEquality& page_equality);
-  void Check(const sessions::SessionTab* tab);
-  void Emit(const PageVisitObserver::TransitionType transition);
-
- private:
-  int Clamp(int input, int lower, int upper);
-
-  const PageEquality page_equality_;
-  const sessions::SessionTab* best_tab_ = nullptr;
-  // Invalid while best_tab_ is nullptr.
-  int best_offset_ = 0;
-
-  DISALLOW_COPY_AND_ASSIGN(OffsetTabMatcher);
-};
-
-}  // namespace sync_driver
-
-#endif  // COMPONENTS_SYNC_DRIVER_OFFSET_TAB_MATCHER_H_