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 15619b6161c35330fd4c4d9d1c56be7506e3b61d..475e450dc48baf98f311a024a2830a1186c24770 100644
--- a/chrome/renderer/autofill/password_autofill_agent_browsertest.cc
+++ b/chrome/renderer/autofill/password_autofill_agent_browsertest.cc
@@ -717,6 +717,19 @@ 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) {

[vabr (Chromium), 2017/06/12 18:56:58]

Could you please also add a negative test? For example for username vs username
+ "suffix_without_@".
The goal is not to test all options (that's done by the unittest), but to test
that the negative path is executed at all.

Furthermore, could you please also add a test that if there is no username
field, and the saved credentials contain an empty username, then the password is
filled properly? The part of code which you are changing handles this event as
well (despite it is not related to prefix matching), and AFAICT, we don't have a
test for that yet.

[melandory, 2017/06/19 10:53:48]

Done
https://codereview.chromium.org/2949533002

+  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.

[vabr (Chromium), 2017/06/12 18:56:58]

nit: "preferred match" is a sort of a term, used in PasswordFormManager to
denote the last used credentials on a form. Here it seems more precise to say:
"...though the username in the form is only a proper prefix of the stored
username"

[melandory, 2017/06/19 10:53:48]

Done.

+  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) {