Let IndexedDBContextImpl create its own task runner

chrome/browser/extensions/app_data_migrator_unittest.cc

 // 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 <stdint.h>
 
 #include <string>
 #include <utility>
 
 #include "base/callback_forward.h"
@@ skipping 35 matching lines @@
   void SetUp() override {
     profile_ = GetTestingProfile();
     registry_ = ExtensionRegistry::Get(profile_.get());
     migrator_ = std::unique_ptr<AppDataMigrator>(
         new AppDataMigrator(profile_.get(), registry_));
 
     default_partition_ =
         content::BrowserContext::GetDefaultStoragePartition(profile_.get());
 
     idb_context_ = default_partition_->GetIndexedDBContext();
-    idb_context_->SetTaskRunnerForTesting(
-        base::ThreadTaskRunnerHandle::Get().get());
 
     default_fs_context_ = default_partition_->GetFileSystemContext();
 
     url_request_context_ = std::unique_ptr<content::MockBlobURLRequestContext>(
         new content::MockBlobURLRequestContext(default_fs_context_));
   }
 
   void TearDown() override {}
 
  protected:
@@ skipping 210 matching lines @@
   content::StoragePartition* new_partition =
       content::BrowserContext::GetStoragePartitionForSite(profile_.get(),
                                                           extension_url);
 
   ASSERT_NE(new_partition->GetPath(), default_partition_->GetPath());
 
   VerifyTestFilesMigrated(new_partition, new_ext.get());
 }
 
 }  // namespace extensions