[PasswordManager] Introduced Finch flag for overwriting placeholders in username fields

To allow for a better impact assessment, the feature to overwrite static placeholders in username fields is retroactively put behind a Finch flag.
Note that older version will always overwrite the static placeholder if the prediction from the server is supplied.

Bug: 1022291
Change-Id: I39f1ac557f7e9751d348b1df5796bfa276839e89
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1903351Reviewed-by: Vadym Doroshenko <dvadym@chromium.org>
Commit-Queue: Matthias Körber <koerber@google.com>
Cr-Commit-Position: refs/heads/master@{#713418}

components/password_manager/core/browser/form_parsing/form_parser.cc

@@ -913,6 +913,10 @@ const FormFieldData* FindUsernameInPredictions(
 bool GetMayUsePrefilledPlaceholder(
     const base::Optional<FormPredictions>& form_predictions,
     const SignificantFields& significant_fields) {
+  if (!base::FeatureList::IsEnabled(
+          password_manager::features::kEnableOverwritingPlaceholderUsernames))
+    return false;
+
   if (!form_predictions || !significant_fields.username)
     return false;
 

components/password_manager/core/browser/form_parsing/form_parser_unittest.cc

@@ -1197,6 +1197,33 @@ TEST(FormParserTest, ServerPredictionsForClearTextPasswordFields) {
   });
 }
 
+TEST(FormParserTest, ServerHintsForDisabledPrefilledPlaceholderFeature) {
+  base::test::ScopedFeatureList feature_list;
+  feature_list.InitAndDisableFeature(
+      password_manager::features::kEnableOverwritingPlaceholderUsernames);
+  CheckTestData({
+      {
+          .description_for_logging = "Simple predictions work",
+          .fields =
+              {
+                  {.role = ElementRole::USERNAME,
+                   .form_control_type = "text",
+                   .prediction = {.type = autofill::USERNAME_AND_EMAIL_ADDRESS,
+                                  .may_use_prefilled_placeholder = true}},
+                  {.form_control_type = "text"},
+                  {.role_saving = ElementRole::CURRENT_PASSWORD,
+                   .form_control_type = "password"},
+                  {.role_filling = ElementRole::CURRENT_PASSWORD,
+                   .role_saving = ElementRole::NEW_PASSWORD,
+                   .form_control_type = "password",
+                   .prediction = {.type = autofill::PASSWORD,
+                                  .may_use_prefilled_placeholder = true}},
+              },
+          .username_may_use_prefilled_placeholder = false,
+      },
+  });
+}
+
 TEST(FormParserTest, ServerHints) {
   CheckTestData({
       {

components/password_manager/core/common/password_manager_features.cc

@@ -20,6 +20,11 @@ const base::Feature kEditPasswordsInDesktopSettings = {
 const base::Feature kDeleteCorruptedPasswords = {
     "DeleteCorruptedPasswords", base::FEATURE_DISABLED_BY_DEFAULT};
 
+// Enables the overwriting of prefilled username fields if the server predicted
+// the field to contain a placeholder value.
+const base::Feature kEnableOverwritingPlaceholderUsernames{
+    "EnableOverwritingPlaceholderUsernames", base::FEATURE_ENABLED_BY_DEFAULT};
+
 // Enables a second, Gaia-account-scoped password store for users who are signed
 // in but not syncing.
 const base::Feature kEnablePasswordsAccountStorage = {

components/password_manager/core/common/password_manager_features.h

@@ -19,6 +19,7 @@ namespace features {
 
 extern const base::Feature kEditPasswordsInDesktopSettings;
 extern const base::Feature kDeleteCorruptedPasswords;
+extern const base::Feature kEnableOverwritingPlaceholderUsernames;
 extern const base::Feature kEnablePasswordsAccountStorage;
 extern const base::Feature KEnablePasswordGenerationForClearTextFields;
 extern const base::Feature kFillOnAccountSelect;