[Sync] Adding revisit logic for typed URLs.

components/sync_sessions/revisit/typed_url_page_revisit_observer.h

Index: components/sync_sessions/revisit/typed_url_page_revisit_observer.h
diff --git a/components/sync_sessions/revisit/typed_url_page_revisit_observer.h b/components/sync_sessions/revisit/typed_url_page_revisit_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..da2d7dd1303b1122bebc9399e0197486ef033a4c
--- /dev/null
+++ b/components/sync_sessions/revisit/typed_url_page_revisit_observer.h
@@ -0,0 +1,43 @@
+// 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_SESSIONS_REVISIT_TYPED_URL_PAGE_REVISIT_OBSERVER_H_
+#define COMPONENTS_SYNC_SESSIONS_REVISIT_TYPED_URL_PAGE_REVISIT_OBSERVER_H_
+
+#include "base/memory/weak_ptr.h"
+#include "base/task/cancelable_task_tracker.h"
+#include "components/sync_sessions/revisit/page_visit_observer.h"
+
+class GURL;
+
+namespace history {
+class HistoryService;
+}  // namespace history
+
+namespace sync_sessions {
+
+class HistoryService;

[stanisc, 2015/10/13 21:53:33]

Second forward declaration for HistoryService. See above.

[skym, 2015/10/13 22:25:14]

Done.

+
+// This class's job is to respond to OnPageVisit events and launch a task to
+// the history thread to perform the requisite checks and instrumentation. It is
+// important that this object is created and called on the same thread, since it
+// creates and owns a weak pointer to the HistoryService.

[stanisc, 2015/10/13 21:53:33]

Nit: it is OK to own a weak pointer to another object on a different thread.
What matters is that dereferencing the weak pointer should be done on the same
thread where it was created.
May be replace "owns" with "uses".

[skym, 2015/10/13 22:25:14]

Done.

+class TypedUrlPageRevisitObserver : public PageVisitObserver {
+ public:
+  explicit TypedUrlPageRevisitObserver(history::HistoryService* history);
+  ~TypedUrlPageRevisitObserver() override;
+  void OnPageVisit(const GURL& url,
+                   const PageVisitObserver::TransitionType transition) override;
+
+ private:
+  const base::WeakPtr<history::HistoryService> history_;
+  // This is never used to cancel tasks, but required by the history interface.
+  base::CancelableTaskTracker task_tracker_;
+
+  DISALLOW_COPY_AND_ASSIGN(TypedUrlPageRevisitObserver);
+};
+
+}  // namespace sync_sessions
+
+#endif  // COMPONENTS_SYNC_SESSIONS_REVISIT_TYPED_URL_PAGE_REVISIT_OBSERVER_H_