Reset user data directory and disk cache directory after downgrade.

chrome/installer/util/install_util.h

 // Copyright (c) 2012 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.
 //
 // This file declares utility functions for the installer. The original reason
 // for putting these functions in installer\util library is so that we can
 // separate out the critical logic and write unit tests for it.
 
 #ifndef CHROME_INSTALLER_UTIL_INSTALL_UTIL_H_
 #define CHROME_INSTALLER_UTIL_INSTALL_UTIL_H_
@@ skipping 165 matching lines @@
   static int GetInstallReturnCode(installer::InstallStatus install_status);
 
   // Composes |program| and |arguments| into |command_line|.
   static void ComposeCommandLine(const base::string16& program,
                                  const base::string16& arguments,
                                  base::CommandLine* command_line);
 
   // Returns a string in the form YYYYMMDD of the current date.
   static base::string16 GetCurrentDate();
 
+  // Return downgrade version if the value exists or empty version.

[grt (UTC plus 2), 2016/05/20 15:25:05]

"Returns..." as per
https://google.github.io/styleguide/cppguide.html#Function_Comments.

also might be nice to explain a bit more about what this value means. how about:

  // Returns the highest Chrome version that was installed prior to a downgrade,
  // or an invalid Version if Chrome was not previously downgraded from a newer
  // version.

[zmin, 2016/05/20 16:33:06]

Done.

+  static base::Version GetDowngradeVersion(HKEY root,
+                                           const BrowserDistribution* dist);
+
+  // Add or remove downgrade version registry value. This function should only

[grt (UTC plus 2), 2016/05/20 15:25:05]

"Adds or removes..."

[zmin, 2016/05/20 16:33:06]

Done.

+  // be used for Chrome install.
+  static void AddUpdateDowngradeVersionItem(
+      HKEY root,
+      const base::Version& current_version,
+      const base::Version& new_version,
+      const BrowserDistribution* dist,
+      WorkItemList* list);
+
   // A predicate that compares the program portion of a command line with a
   // given file path.  First, the file paths are compared directly.  If they do
   // not match, the filesystem is consulted to determine if the paths reference
   // the same file.
   class ProgramCompare : public RegistryValuePredicate {
    public:
     enum class ComparisonType {
       // Evaluation compares existing files.
       FILE,
       // Evaluation compares existing files or directories.
@@ skipping 26 matching lines @@
    private:
     DISALLOW_COPY_AND_ASSIGN(ProgramCompare);
   };  // class ProgramCompare
 
  private:
   DISALLOW_COPY_AND_ASSIGN(InstallUtil);
 };
 
 
 #endif  // CHROME_INSTALLER_UTIL_INSTALL_UTIL_H_