Reset user data directory and disk cache directory after downgrade.

chrome/installer/util/install_util.h

Index: chrome/installer/util/install_util.h
diff --git a/chrome/installer/util/install_util.h b/chrome/installer/util/install_util.h
index 79b8e2a2304f8755ce5a038b18da275ec1ddd017..84ab97718bb421900897f6132e0bdd22ef50941f 100644
--- a/chrome/installer/util/install_util.h
+++ b/chrome/installer/util/install_util.h
@@ -183,6 +183,19 @@ class InstallUtil {
   // 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