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..ff76fc66f999f80c40683ed963c5483629c45bc8 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"DowngradeVersion";
 const wchar_t kStageBinaryPatching[] = L"binary_patching";
 const wchar_t kStageBuilding[] = L"building";
 const wchar_t kStageConfiguringAutoLaunch[] = L"configuring_auto_launch";
@@ -627,6 +628,46 @@ bool InstallUtil::ProgramCompare::GetInfo(const base::File& file,
   return GetFileInformationByHandle(file.GetPlatformFile(), info) != 0;
 }
 
+// static
+base::Version InstallUtil::GetDowngradeVersion(
+    bool system_install,
+    const BrowserDistribution* dist) {
+  DCHECK(dist);
+  base::win::RegKey key;
+  base::string16 downgrade_version;
+  if (key.Open(system_install ? HKEY_LOCAL_MACHINE : HKEY_CURRENT_USER,
+               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(
+    bool system_install,
+    const base::Version* current_version,
+    const base::Version& new_version,
+    const BrowserDistribution* dist,
+    WorkItemList* list) {
+  DCHECK(list);
+  DCHECK(dist);

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

to enforce the "only call this for Chrome", add:
  DCHECK_EQ(BrowserDistribution::CHROME_BROWSER, dist->GetType());

[zmin, 2016/05/20 19:11:21]

Done.

+  base::Version downgrade_version = GetDowngradeVersion(system_install, dist);
+  HKEY root = system_install ? HKEY_LOCAL_MACHINE : HKEY_CURRENT_USER;
+  if (current_version == nullptr ||

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

nit: if (!current_version

[zmin, 2016/05/20 19:11:21]

Done.

+      (*current_version < new_version &&

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

should this be <= so that the delete is done for same-version-repair as well? is
there harm in this?

[zmin, 2016/05/20 19:11:21]

For same-version-repair, the |downgrade_version| should be taken care of by
previous install. However, because "strange things happen in the registry on a
regular basis", I think it worth to add '=' so that we can fix the
|downgrade_version| during repair if needed.

+       ((!downgrade_version.IsValid() || downgrade_version <= new_version)))) {
+    list->AddDeleteRegValueWorkItem(root, dist->GetStateKey(), KEY_WOW64_32KEY,
+                                    kRegDowngradeVersion);
+  } else if (*current_version > new_version && !downgrade_version.IsValid()) {
+    list->AddSetRegValueWorkItem(
+        root, dist->GetStateKey(), KEY_WOW64_32KEY, kRegDowngradeVersion,
+        base::ASCIIToUTF16(current_version->GetString()), true);
+  }
+}
+
 InstallUtil::ProgramCompare::ProgramCompare(const base::FilePath& path_to_match)
     : ProgramCompare(path_to_match, ComparisonType::FILE) {}