Test that LoginDatabase preserves PasswordForm::Scheme through migration

components/password_manager/core/browser/login_database_win.cc

Index: components/password_manager/core/browser/login_database_win.cc
diff --git a/components/password_manager/core/browser/login_database_win.cc b/components/password_manager/core/browser/login_database_win.cc
index 2b2b2725573c21466fb57fe6cb1a674e94548c9a..6cf9c2188e789b31ea81ce7c105d1e34afcec5e1 100644
--- a/components/password_manager/core/browser/login_database_win.cc
+++ b/components/password_manager/core/browser/login_database_win.cc
@@ -22,6 +22,19 @@ LoginDatabase::EncryptionResult LoginDatabase::EncryptedString(
 LoginDatabase::EncryptionResult LoginDatabase::DecryptedString(
     const std::string& cipher_text,
     base::string16* plain_text) {
+  // Unittests need to read sample database entries. If these entries had real
+  // passwords, their encoding would need to be different for every platform.
+  // To avoid the need for that, the entries have empty passwords. OSCrypt on
+  // Windows does not recognise the empty string as a valid encrypted string.
+  // Changing that for all clients of OSCrypt could have too broad an impact,
+  // therefore to allow platform-independent data files for LoginDatabase
+  // tests, the special handling of the empty string is added below instead.
+  // See also https://codereview.chromium.org/2291123008/#msg14 for a
+  // discussion.
+  if (cipher_text.empty()) {
+    plain_text->clear();
+    return ENCRYPTION_RESULT_SUCCESS;
+  }
   if (OSCrypt::DecryptString16(cipher_text, plain_text))
     return ENCRYPTION_RESULT_SUCCESS;
   return ENCRYPTION_RESULT_ITEM_FAILURE;