[Sync] Update DirectoryBackingStore to detect Sync DB corruption

sync/syncable/deferred_on_disk_directory_backing_store_unittest.cc

 // Copyright 2014 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 "base/files/scoped_temp_dir.h"
+#include "base/message_loop/message_loop.h"
 #include "base/stl_util.h"
 #include "sync/syncable/deferred_on_disk_directory_backing_store.h"
 #include "sync/syncable/directory.h"
 #include "sync/syncable/entry_kernel.h"
 #include "sync/syncable/on_disk_directory_backing_store.h"
 #include "sync/syncable/syncable_delete_journal.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace syncer {
 namespace syncable {
 namespace {
 
 static const base::FilePath::CharType kSyncDataFolderName[] =
     FILE_PATH_LITERAL("Sync Data");
 
 class DeferredOnDiskDirectoryBackingStoreTest : public testing::Test {
  protected:
   void SetUp() override {
     CHECK(temp_dir_.CreateUniqueTempDir());
     db_path_ = temp_dir_.path().Append(kSyncDataFolderName);
   }
 
   void TearDown() override { STLDeleteValues(&handles_map_); }
 
+  base::MessageLoop message_loop_;
   base::ScopedTempDir temp_dir_;
   base::FilePath db_path_;
   Directory::MetahandlesMap handles_map_;
   JournalIndex delete_journals_;
   MetahandleSet metahandles_to_purge_;
   Directory::KernelLoadInfo kernel_load_info_;
 };
 
 // Test initialization of root entry when calling Load().
 TEST_F(DeferredOnDiskDirectoryBackingStoreTest, Load) {
@@ skipping 66 matching lines @@
   ASSERT_EQ(2u, handles_map_.size());
   ASSERT_TRUE(handles_map_.count(1));     // root node
   ASSERT_TRUE(handles_map_.count(2));
   EntryKernel* entry = handles_map_[2];
   EXPECT_EQ(Id::CreateFromClientString("test_entry"), entry->ref(ID));
 }
 
 }  // namespace
 }  // namespace syncable
 }  // namespace syncer