Reset user data directory and disk cache directory after downgrade.

chrome/installer/util/google_update_constants.cc

Index: chrome/installer/util/google_update_constants.cc
diff --git a/chrome/installer/util/google_update_constants.cc b/chrome/installer/util/google_update_constants.cc
index e853863aa52fa4abb326909b81b3adc896713c15..e2400a95b8dcc74e00b51015875c96ace5eb28dd 100644
--- a/chrome/installer/util/google_update_constants.cc
+++ b/chrome/installer/util/google_update_constants.cc
@@ -34,6 +34,7 @@ const wchar_t kRegClientField[] = L"client";
 const wchar_t kRegCommandLineField[] = L"CommandLine";
 const wchar_t kRegCriticalVersionField[] = L"cpv";
 const wchar_t kRegDidRunField[] = L"dr";
+const wchar_t kRegDowngradeVersion[] = L"DowngradeVersion";
 const wchar_t kRegEULAAceptedField[] = L"eulaaccepted";
 const wchar_t kRegGoogleUpdateVersion[] = L"version";
 const wchar_t kRegLangField[] = L"lang";