Reset user data directory and disk cache directory after downgrade.

chrome/browser/downgrade/user_data_downgrade_browsertest.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 "chrome/browser/downgrade/user_data_downgrade.h"
+
+#include "base/files/file_util.h"
+#include "base/path_service.h"
+#include "base/version.h"
+#include "chrome/common/chrome_constants.h"
+#include "chrome/common/chrome_paths.h"
+#include "chrome/test/base/in_process_browser_test.h"
+#include "content/public/browser/browser_thread.h"
+
+class UserDataDowngradeBrowserTest : public InProcessBrowserTest {
+ protected:
+  void SetUpInProcessBrowserTestFixture() override { EnableBrowserTest(); }
+
+  void TearDownInProcessBrowserTestFixture() override {
+    const base::FilePath::StringType delete_suffix =

[grt (UTC plus 2), 2016/05/27 16:26:28]

nit: constexpr

[grt (UTC plus 2), 2016/05/27 16:27:04]

oops, nevermind. i read it as ::CharType rather than ::StringType

+        FILE_PATH_LITERAL(" (1).CHROME_DELETE");
+    ASSERT_FALSE(base::DirectoryExists(
+        base::FilePath(user_data_dir_.value() + delete_suffix)));
+  }
+
+  bool SetUpUserDataDirectory() override {
+    const base::FilePath::StringType last_chrome_version_file =
+        FILE_PATH_LITERAL("Last Chrome Version");
+    if (!PathService::Get(chrome::DIR_USER_DATA, &user_data_dir_))
+      return false;
+    if (!CreateTemporaryFileInDir(user_data_dir_, &other_file_))
+      return false;
+    last_chrome_version_path_ = user_data_dir_.Append(last_chrome_version_file);
+    std::string last_chrome_version =
+        base::Version(std::string(chrome::kChromeVersion) + "1").GetString();
+    base::WriteFile(last_chrome_version_path_, last_chrome_version.c_str(),
+                    last_chrome_version.size());
+    return true;
+  }
+
+  base::FilePath last_chrome_version_path_;
+  base::FilePath other_file_;
+  base::FilePath user_data_dir_;
+};
+
+IN_PROC_BROWSER_TEST_F(UserDataDowngradeBrowserTest, CopyAndClean) {
+  std::string last_chrome_version_str;
+  base::ReadFileToString(last_chrome_version_path_, &last_chrome_version_str);

[grt (UTC plus 2), 2016/05/27 16:26:28]

please add comments to explain what this is testing

[zmin, 2016/05/27 17:43:00]

Done.

+  EXPECT_EQ(chrome::kChromeVersion, last_chrome_version_str);
+  ASSERT_FALSE(base::PathExists(other_file_));
+}