Enable manual password autofilling if the username field is read-only.

components/autofill/content/renderer/password_autofill_agent.cc

Index: components/autofill/content/renderer/password_autofill_agent.cc
diff --git a/components/autofill/content/renderer/password_autofill_agent.cc b/components/autofill/content/renderer/password_autofill_agent.cc
index 34e28598f3145eb9c75ac0ff73b0ef58e46d428d..61f31211bb530992057dab8fbe08c0f7c0b0e64d 100644
--- a/components/autofill/content/renderer/password_autofill_agent.cc
+++ b/components/autofill/content/renderer/password_autofill_agent.cc
@@ -703,8 +703,6 @@ bool PasswordAutofillAgent::FillSuggestion(
 
   if (!FindPasswordInfoForElement(*element, &username_element,
                                   &password_element, &password_info) ||
-      (!username_element.isNull() &&
-       !IsElementAutocompletable(username_element)) ||
       !IsElementAutocompletable(password_element)) {
     return false;
   }
@@ -713,7 +711,8 @@ bool PasswordAutofillAgent::FillSuggestion(
   if (element->isPasswordField()) {
     password_info->password_field_suggestion_was_accepted = true;
     password_info->password_field = password_element;
-  } else if (!username_element.isNull()) {
+  } else if (!username_element.isNull() &&
+             IsElementAutocompletable(username_element)) {
     username_element.setValue(blink::WebString(username), true);
     username_element.setAutofilled(true);
     UpdateFieldValueAndPropertiesMaskMap(username_element, &username,
@@ -749,13 +748,12 @@ bool PasswordAutofillAgent::PreviewSuggestion(
 
   if (!FindPasswordInfoForElement(*element, &username_element,
                                   &password_element, &password_info) ||
-      (!username_element.isNull() &&
-       !IsElementAutocompletable(username_element)) ||
       !IsElementAutocompletable(password_element)) {
     return false;
   }
 
-  if (!element->isPasswordField() && !username_element.isNull()) {
+  if (!element->isPasswordField() && !username_element.isNull() &&
+      IsElementAutocompletable(username_element)) {
     if (username_query_prefix_.empty())
       username_query_prefix_ = username_element.value();