[Sync] Creating sync_sessions component, moving revisit logic.

components/sync_driver/revisit/page_visit_observer.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_REVISIT_PAGE_VISIT_OBSERVERE_H_
-#define COMPONENTS_SYNC_DRIVER_REVISIT_PAGE_VISIT_OBSERVERE_H_
-
-#include "url/gurl.h"
-
-namespace sync_driver {
-
-// An interface that allows observers to be notified when a page is visited.
-class PageVisitObserver {
- public:
-  // This enum represents the most common ways to visit a new page/URL.
-  enum TransitionType {
-    kTransitionPage = 0,
-    kTransitionOmniboxUrl = 1,
-    kTransitionOmniboxDefaultSearch = 2,
-    kTransitionOmniboxTemplateSearch = 3,
-    kTransitionBookmark = 4,
-    kTransitionCopyPaste = 5,
-    kTransitionForwardBackward = 6,
-    kTransitionRestore = 7,
-    kTransitionUnknown = 8,
-    kTransitionTypeLast = 9,
-  };
-
-  virtual ~PageVisitObserver() {}
-  virtual void OnPageVisit(const GURL& url,
-                           const TransitionType transition) = 0;
-};
-
-}  // namespace sync_driver
-
-#endif  // COMPONENTS_SYNC_DRIVER_REVISIT_PAGE_VISIT_OBSERVERE_H_