Add Store+Sync to reading list.

ios/chrome/browser/reading_list/reading_list_model.cc

 // Copyright 2016 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 "ios/chrome/browser/reading_list/reading_list_model.h"
 
+#include "base/logging.h"
+#include "ios/web/public/web_thread.h"
+
 ReadingListModel::ReadingListModel() : current_batch_updates_count_(0) {}
+
 ReadingListModel::~ReadingListModel() {
   for (auto& observer : observers_) {
     observer.ReadingListModelBeingDeleted(this);
   }
 }
 
 // Observer methods.
 void ReadingListModel::AddObserver(ReadingListModelObserver* observer) {
+  DCHECK(CalledOnValidThread());
   DCHECK(observer);
   observers_.AddObserver(observer);
   if (loaded()) {
     observer->ReadingListModelLoaded(this);
   }
 }
 
 void ReadingListModel::RemoveObserver(ReadingListModelObserver* observer) {
+  DCHECK(CalledOnValidThread());
   observers_.RemoveObserver(observer);
 }
 
 // Batch update methods.
 bool ReadingListModel::IsPerformingBatchUpdates() const {
+  DCHECK(CalledOnValidThread());
   return current_batch_updates_count_ > 0;
 }
 
 std::unique_ptr<ReadingListModel::ScopedReadingListBatchUpdate>
 ReadingListModel::BeginBatchUpdates() {
+  DCHECK(CalledOnValidThread());
   std::unique_ptr<ReadingListModel::ScopedReadingListBatchUpdate> token(
       new ReadingListModel::ScopedReadingListBatchUpdate(this));
 
   ++current_batch_updates_count_;
   if (current_batch_updates_count_ == 1) {
-    for (auto& observer : observers_)
-      observer.ReadingListModelBeganBatchUpdates(this);
+    EnteringBatchUpdates();
   }
   return token;
 }
 
+void ReadingListModel::EnteringBatchUpdates() {
+  DCHECK(CalledOnValidThread());
+  for (auto& observer : observers_)
+    observer.ReadingListModelBeganBatchUpdates(this);
+}
+
 void ReadingListModel::EndBatchUpdates() {
+  DCHECK(CalledOnValidThread());
   DCHECK(IsPerformingBatchUpdates());
+  DCHECK(current_batch_updates_count_ > 0);
   --current_batch_updates_count_;
   if (current_batch_updates_count_ == 0) {
-    for (auto& observer : observers_)
-      observer.ReadingListModelCompletedBatchUpdates(this);
+    LeavingBatchUpdates();
   }
 }
+
+void ReadingListModel::LeavingBatchUpdates() {
+  DCHECK(CalledOnValidThread());
+  for (auto& observer : observers_)
+    observer.ReadingListModelCompletedBatchUpdates(this);
+}