Teach PasswordAutofillAgent sometimes match prefixes of usernames

chrome/renderer/autofill/password_autofill_agent_browsertest.cc

Index: chrome/renderer/autofill/password_autofill_agent_browsertest.cc
diff --git a/chrome/renderer/autofill/password_autofill_agent_browsertest.cc b/chrome/renderer/autofill/password_autofill_agent_browsertest.cc
index f6d1a25446a6be6efba8edb24495bb3c9931251c..37621b9bd544024d17147b5698396edaa4a70352 100644
--- a/chrome/renderer/autofill/password_autofill_agent_browsertest.cc
+++ b/chrome/renderer/autofill/password_autofill_agent_browsertest.cc
@@ -694,6 +694,19 @@ TEST_F(PasswordAutofillAgentTest,
                        UTF16ToUTF8(password3_), true);
 }
 
+// Fill a password field if the stored username is a prefix of username in
+// read-only field.
+TEST_F(PasswordAutofillAgentTest,
+       AutocompletePasswordForReadonlyUsernamePrefixMatched) {
+  base::string16 username_at = username3_ + base::UTF8ToUTF16("@example.com");
+  username_element_.SetValue(WebString::FromUTF16(username3_));
+  SetElementReadOnly(username_element_, true);
+
+  // Filled even though username is not the preferred match.
+  SimulateOnFillPasswordForm(fill_data_);
+  CheckTextFieldsState(UTF16ToUTF8(username3_), false, UTF16ToUTF8(password3_),
+                       true);
+}
 // If a username field is empty and readonly, don't autofill.
 TEST_F(PasswordAutofillAgentTest,
        NoAutocompletePasswordForReadonlyUsernameUnmatched) {