Create a TestFileSystemBackend in incognito test provider

Modifies CreateIncognitoFileSystemContextForTesting method

Bug: N/A
Change-Id: I28a86cb4edb35d312899ac438e283bd502fd1722
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2487723Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Austin Sullivan <asully@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819416}

storage/browser/test/test_file_system_context.cc

@@ -66,6 +66,8 @@ FileSystemContext* CreateIncognitoFileSystemContextForTesting(
     QuotaManagerProxy* quota_manager_proxy,
     const base::FilePath& base_path) {
   std::vector<std::unique_ptr<FileSystemBackend>> additional_providers;
+  additional_providers.push_back(std::make_unique<TestFileSystemBackend>(
+      base::ThreadTaskRunnerHandle::Get().get(), base_path));
   return CreateIncognitoFileSystemContextWithAdditionalProvidersForTesting(
       io_task_runner, file_task_runner, quota_manager_proxy,
       std::move(additional_providers), base_path);