Reset user data directory and disk cache directory after downgrade.

chrome/installer/util/install_util.cc

Index: chrome/installer/util/install_util.cc
diff --git a/chrome/installer/util/install_util.cc b/chrome/installer/util/install_util.cc
index 8b6dfa75deab255b214ff5e749ab7e76949ad6a7..726947bd87046a4ab067705d294b3e4e7aa47c69 100644
--- a/chrome/installer/util/install_util.cc
+++ b/chrome/installer/util/install_util.cc
@@ -44,6 +44,7 @@ using installer::ProductState;
 
 namespace {
 
+const wchar_t kRegDowngradeVersion[] = L"DowngrdeVersion";
 const wchar_t kStageBinaryPatching[] = L"binary_patching";
 const wchar_t kStageBuilding[] = L"building";
 const wchar_t kStageConfiguringAutoLaunch[] = L"configuring_auto_launch";
@@ -627,6 +628,43 @@ bool InstallUtil::ProgramCompare::GetInfo(const base::File& file,
   return GetFileInformationByHandle(file.GetPlatformFile(), info) != 0;
 }
 
+// static
+base::Version InstallUtil::GetDowngradeVersion(
+    HKEY root,
+    const BrowserDistribution* dist) {
+  DCHECK(dist);
+  base::win::RegKey key;
+  base::string16 downgrade_version;
+  if (key.Open(root, dist->GetStateKey().c_str(),
+               KEY_QUERY_VALUE | KEY_WOW64_32KEY) != ERROR_SUCCESS ||
+      key.ReadValue(kRegDowngradeVersion, &downgrade_version) !=
+          ERROR_SUCCESS) {
+    return base::Version();
+  }
+  return base::Version(base::UTF16ToASCII(downgrade_version));
+}
+
+// static
+void InstallUtil::AddUpdateDowngradeVersionItem(
+    HKEY root,
+    const base::Version& current_version,
+    const base::Version& new_version,
+    const BrowserDistribution* dist,
+    WorkItemList* list) {
+  DCHECK(list);
+  DCHECK(dist);
+  base::Version downgrade_version = GetDowngradeVersion(root, dist);
+  if (current_version > new_version && !downgrade_version.IsValid()) {
+    list->AddSetRegValueWorkItem(
+        root, dist->GetStateKey(), KEY_WOW64_32KEY, kRegDowngradeVersion,
+        base::ASCIIToUTF16(current_version.GetString()), false);

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

false -> true since you want to overwrite if there happens to be bogus data in
the value.

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

Done.

+  } else if (current_version < new_version && downgrade_version.IsValid() &&

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

|downgrade_version| will be invalid if either the value is not present in the
registry, or if it a bogus value is present. i think it's desirable to delete a
bogus value. with that in mind, what do you think about changing this to:
  } else if (current_version < new_version &&
             (!downgrade_version.IsValid() ||
              downgrade_version <= new_version)) {

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

That means for every upgrade(even for the ppl never downgrade chrome), we'll try
to delete a value that never existed. If bogus value is something happened
before for other registry value and the cost of deletion is tiny, I'll agree
with that.

[grt (UTC plus 2), 2016/05/20 18:17:05]

the cost of the delete is insignificant. strange things happen in the registry
on a regular basis, so it's best to be defensive.

+             downgrade_version <= new_version) {
+    list->AddDeleteRegValueWorkItem(root, dist->GetStateKey(), KEY_WOW64_32KEY,
+                                    kRegDowngradeVersion);
+  }
+}
+
 InstallUtil::ProgramCompare::ProgramCompare(const base::FilePath& path_to_match)
     : ProgramCompare(path_to_match, ComparisonType::FILE) {}