[Sync] Creating sync_sessions component, moving revisit logic.

components/sync_driver/revisit/sessions_page_revisit_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_SESSIONS_PAGE_REVISIT_OBSERVER_H_
-#define COMPONENTS_SYNC_DRIVER_REVISIT_SESSIONS_PAGE_REVISIT_OBSERVER_H_
-
-#include <vector>
-
-#include "base/memory/scoped_ptr.h"
-#include "base/memory/weak_ptr.h"
-#include "components/sync_driver/revisit/page_visit_observer.h"
-#include "url/gurl.h"
-
-namespace sessions {
-struct SessionTab;
-}  // namespace sessions
-
-namespace sync_driver {
-
-struct SyncedSession;
-
-class CurrentTabMatcher;
-class OffsetTabMatcher;
-
-// A simple interface to abstract away who is providing sessions.
-class ForeignSessionsProvider {
- public:
-  // Fills the already instantiated passed vector with all foreign sessions.
-  // Returned boolean representes if there were foreign sessions and the vector
-  // should be examimed.
-  virtual bool GetAllForeignSessions(
-      std::vector<const sync_driver::SyncedSession*>* sessions) = 0;
-  virtual ~ForeignSessionsProvider() {}
-};
-
-// An implementation of PageVisitObserver that checks the given page's url
-// against in memory session information to detect if we've seen this page
-// before, constituting a revisit. Then histogram information is emitted about
-// this page navigation.
-class SessionsPageRevisitObserver
-    : public PageVisitObserver,
-      public base::SupportsWeakPtr<SessionsPageRevisitObserver> {
- public:
-  explicit SessionsPageRevisitObserver(
-      scoped_ptr<ForeignSessionsProvider> provider);
-  ~SessionsPageRevisitObserver() override;
-  void OnPageVisit(const GURL& url, const TransitionType transition) override;
-
- private:
-  friend class SessionsPageRevisitObserverTest;
-
-  // Although the signature is identical to OnPageVisit(...), this method
-  // actually does all of the work. The assumption is that this method is the
-  // target of a PostTask call coming from OnPageVisit(...).
-  void CheckForRevisit(const GURL& url, const TransitionType transition);
-
-  scoped_ptr<ForeignSessionsProvider> provider_;
-
-  DISALLOW_COPY_AND_ASSIGN(SessionsPageRevisitObserver);
-};
-
-}  // namespace sync_driver
-
-#endif  // COMPONENTS_SYNC_DRIVER_REVISIT_SESSIONS_PAGE_REVISIT_OBSERVER_H_