Use base::OnTaskRunnerDeleter in HistoryModelWorker.

base::OnTaskRunnerDeleter is the preferred way of ensuring that
a member is destroyed on a specific sequence.

Change-Id: I86dde3b343f03201afdde3caed1e84762e55c1cb
Reviewed-on: https://chromium-review.googlesource.com/977888
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546552}

components/history/core/browser/history_model_worker.cc

@@ -53,11 +53,17 @@ void PostWorkerTask(
 
 HistoryModelWorker::HistoryModelWorker(
     const base::WeakPtr<history::HistoryService>& history_service,
-    const scoped_refptr<base::SingleThreadTaskRunner>& ui_thread)
-    : history_service_(history_service), ui_thread_(ui_thread) {
+    scoped_refptr<base::SingleThreadTaskRunner> ui_thread)
+    : history_service_(history_service),
+      ui_thread_(std::move(ui_thread)),
+      // base::OnTaskRunnerDeleter ensures that |cancelable_tracker_| is
+      // destroyed on the UI thread, even if |this| is deleted on a different
+      // thread. It is a requirement of base::CancelableTaskTracker to be
+      // constructed and deleted on the same sequence.
+      cancelable_tracker_(new base::CancelableTaskTracker,
+                          base::OnTaskRunnerDeleter(ui_thread_)) {
   CHECK(history_service.get());
   DCHECK(ui_thread_->BelongsToCurrentThread());
-  cancelable_tracker_.reset(new base::CancelableTaskTracker);
 }
 
 syncer::ModelSafeGroup HistoryModelWorker::GetModelSafeGroup() {
@@ -71,12 +77,7 @@ bool HistoryModelWorker::IsOnModelSequence() {
   return true;
 }
 
-HistoryModelWorker::~HistoryModelWorker() {
-  // The base::CancelableTaskTracker class is not thread-safe and must only be
-  // used from a single thread but the current object may not be destroyed from
-  // the UI thread, so delete it from the UI thread.
-  ui_thread_->DeleteSoon(FROM_HERE, cancelable_tracker_.release());
-}
+HistoryModelWorker::~HistoryModelWorker() = default;
 
 void HistoryModelWorker::ScheduleWork(base::OnceClosure work) {
   ui_thread_->PostTask(

components/history/core/browser/history_model_worker.h

@@ -26,7 +26,7 @@ class HistoryModelWorker : public syncer::ModelSafeWorker {
  public:
   explicit HistoryModelWorker(
       const base::WeakPtr<history::HistoryService>& history_service,
-      const scoped_refptr<base::SingleThreadTaskRunner>& ui_thread);
+      scoped_refptr<base::SingleThreadTaskRunner> ui_thread);
 
   // syncer::ModelSafeWorker implementation.
   syncer::ModelSafeGroup GetModelSafeGroup() override;
@@ -44,7 +44,8 @@ class HistoryModelWorker : public syncer::ModelSafeWorker {
 
   // Helper object to make sure we don't leave tasks running on the history
   // thread.
-  std::unique_ptr<base::CancelableTaskTracker> cancelable_tracker_;
+  const std::unique_ptr<base::CancelableTaskTracker, base::OnTaskRunnerDeleter>
+      cancelable_tracker_;
 
   DISALLOW_COPY_AND_ASSIGN(HistoryModelWorker);
 };