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/strings/utf_string_conversions.h"
+#include "base/test/test_reg_util_win.h"
+#include "base/version.h"
+#include "base/win/registry.h"
+#include "chrome/common/chrome_constants.h"
+#include "chrome/common/chrome_paths.h"
+#include "chrome/installer/util/browser_distribution.h"
+#include "chrome/test/base/in_process_browser_test.h"
+#include "content/public/browser/browser_thread.h"
+
+class UserDataDowngradeBrowserTestBase : public InProcessBrowserTest {
+ protected:
+  // Verify the renamed user data directory has been deleted.

[grt (UTC plus 2), 2016/06/07 01:25:28]

it's common to document the class that declares overidden methods like so:
  // content::BrowserTestBase:

[zmin, 2016/06/07 14:41:49]

Done. Also did to SetUpInProcessBrowserTestFixture() below.

+  void TearDownInProcessBrowserTestFixture() override {
+    const base::FilePath::StringType index_suffix = FILE_PATH_LITERAL(" (1)");
+    ASSERT_FALSE(base::DirectoryExists(
+        base::FilePath(user_data_dir_.value() + index_suffix)
+            .AddExtension(g_downgrade_delete_suffix)));
+    SimulateDowngradeForTest(false);
+  }
+
+  bool SetUpUserDataDirectory() override {

[grt (UTC plus 2), 2016/06/07 01:25:28]

  // InProcessBrowserTest:

also, please move this up above the TearDown method above so that they're listed
in the order they're invoked

[zmin, 2016/06/07 14:41:49]

Done.

+    if (!PathService::Get(chrome::DIR_USER_DATA, &user_data_dir_))
+      return false;
+    if (!CreateTemporaryFileInDir(user_data_dir_, &other_file_))
+      return false;
+    base::FilePath browser_version_file_path =
+        user_data_dir_.Append(g_downgrade_browser_version_file);
+    std::string last_browser_version = GetNextChromeVersion();
+    base::WriteFile(browser_version_file_path, last_browser_version.c_str(),
+                    last_browser_version.size());
+    return true;
+  }
+
+  std::string GetNextChromeVersion() {
+    return base::Version(std::string(chrome::kChromeVersion) + "1").GetString();
+  }
+
+  base::FilePath other_file_;

[grt (UTC plus 2), 2016/06/07 01:25:28]

please document. maybe something like:
  // The path to an arbitrary file in the user data dir that will be present
only when a reset does not take place.

[zmin, 2016/06/07 14:41:49]

Done.

+  base::FilePath user_data_dir_;
+  registry_util::RegistryOverrideManager registry_override_manager_;
+};
+
+class UserDataDowngradeBrowserCopyAndCleanTest
+    : public UserDataDowngradeBrowserTestBase {
+  void SetUpInProcessBrowserTestFixture() override {
+    SimulateDowngradeForTest(true);
+    HKEY root = HKEY_CURRENT_USER;
+    registry_override_manager_.OverrideRegistry(root);
+    base::win::RegKey key(
+        root, BrowserDistribution::GetDistribution()->GetStateKey().c_str(),
+        KEY_SET_VALUE | KEY_WOW64_32KEY);
+    key.WriteValue(L"DowngradeVersion",
+                   base::ASCIIToUTF16(GetNextChromeVersion()).c_str());
+  }
+};
+
+class UserDataDowngradeBrowserNoResetTest
+    : public UserDataDowngradeBrowserTestBase {
+  void SetUpInProcessBrowserTestFixture() override {
+    SimulateDowngradeForTest(true);
+  }
+};
+
+// Verify the user data directory has been renamed and created again after
+// downgrade.
+IN_PROC_BROWSER_TEST_F(UserDataDowngradeBrowserCopyAndCleanTest, Test) {
+  EXPECT_EQ(chrome::kChromeVersion,
+            GetBrowserVersion(user_data_dir_).GetString());
+  ASSERT_FALSE(base::PathExists(other_file_));
+}
+
+// Verify the user data directory will not be reset without downgrade.
+IN_PROC_BROWSER_TEST_F(UserDataDowngradeBrowserNoResetTest, Test) {
+  EXPECT_EQ(chrome::kChromeVersion,
+            GetBrowserVersion(user_data_dir_).GetString());
+  ASSERT_TRUE(base::PathExists(other_file_));
+}