[Sync] Adding revisit logic for typed URLs.

components/sync_sessions/revisit/typed_url_page_revisit_observer.cc

Index: components/sync_sessions/revisit/typed_url_page_revisit_observer.cc
diff --git a/components/sync_sessions/revisit/typed_url_page_revisit_observer.cc b/components/sync_sessions/revisit/typed_url_page_revisit_observer.cc
new file mode 100644
index 0000000000000000000000000000000000000000..159189577cee28a3691d82e8d9e7c03e44d4067c
--- /dev/null
+++ b/components/sync_sessions/revisit/typed_url_page_revisit_observer.cc
@@ -0,0 +1,30 @@
+// 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.
+
+#include "components/sync_sessions/revisit/typed_url_page_revisit_observer.h"
+
+#include "base/memory/scoped_ptr.h"
+#include "components/history/core/browser/history_service.h"
+#include "components/sync_sessions/revisit/typed_url_page_revisit_task.h"
+#include "url/gurl.h"
+
+namespace sync_sessions {
+
+TypedUrlPageRevisitObserver::TypedUrlPageRevisitObserver(
+    history::HistoryService* history)
+    : history_(base::AsWeakPtr(history)) {}
+
+TypedUrlPageRevisitObserver::~TypedUrlPageRevisitObserver() {}
+
+void TypedUrlPageRevisitObserver::OnPageVisit(
+    const GURL& url,
+    const PageVisitObserver::TransitionType transition) {
+  if (history_) {
+    history_->ScheduleDBTask(scoped_ptr<history::HistoryDBTask>(
+                                 new TypedUrlPageRevisitTask(url, transition)),
+                             &task_tracker_);

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

Is that OK to reuse task_tracker_? What if the previous task hasn't finished?
Could it be cancelled as a result?

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

Great question. It looks like it is okay, it seems that the
CancelableTaskTracker object is essentially a map of ids to flags, see
https://code.google.com/p/chromium/codesearch#chromium/src/base/task/cancelab...
, so having multiple outstanding tasks is pretty much its job.

You can see in this code path history logic requests an id from the tracker at
https://code.google.com/p/chromium/codesearch#chromium/src/components/history...
which is basically just ++ing an int64
https://code.google.com/p/chromium/codesearch#chromium/src/base/task/cancelab...
, so each concurrently running task will have a unique task id (assuming that we
spawn tasks on the same thread, which we do, UI thread).

+  }
+}
+
+}  // namespace sync_sessions