Check multiple roles in form_parser_unittest.cc

The test expectations for form_parser_unittest.cc contain the expected
roles for form fields (username, new-password, etc.). There can be at
most one of each, so if there are multiple same roles, all but one are
ignored.

This CL adds a DCHECK to guard that there are no multiple same roles
specified, so that the expecations are not described in a confusing
way. It is a DCHECK instead of an ASSERT/EXPECT, because this is
something not depending on the tested code, just the way the test is
written. Hence if it passes when submitting the CL, it will pass until
the next change to the test file.

Bug: 902700
Change-Id: I022daf5a1701f13e7d52c881b60acfc412cd2e96
Reviewed-on: https://chromium-review.googlesource.com/c/1325985Reviewed-by: Vadym Doroshenko <dvadym@chromium.org>
Commit-Queue: Vaclav Brozek <vabr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606414}

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

@@ -124,20 +124,25 @@ struct ParseResultIds {
 void UpdateResultWithIdByRole(ParseResultIds* result,
                               uint32_t id,
                               ElementRole role) {
+  constexpr uint32_t kUnassigned = FormFieldData::kNotSetFormControlRendererId;
   switch (role) {
     case ElementRole::NONE:
       // Nothing to update.
       break;
     case ElementRole::USERNAME:
+      DCHECK_EQ(kUnassigned, result->username_id);
       result->username_id = id;
       break;
     case ElementRole::CURRENT_PASSWORD:
+      DCHECK_EQ(kUnassigned, result->password_id);
       result->password_id = id;
       break;
     case ElementRole::NEW_PASSWORD:
+      DCHECK_EQ(kUnassigned, result->new_password_id);
       result->new_password_id = id;
       break;
     case ElementRole::CONFIRMATION_PASSWORD:
+      DCHECK_EQ(kUnassigned, result->confirmation_password_id);
       result->confirmation_password_id = id;
       break;
   }
@@ -1581,8 +1586,7 @@ TEST(FormParserTest, MultipleUsernames) {
               {.role_filling = ElementRole::USERNAME,
                .form_control_type = "text",
                .prediction = {.type = autofill::USERNAME}},
-              {.role = ElementRole::NEW_PASSWORD,
-               .form_control_type = "password",
+              {.form_control_type = "password",
                .prediction = {.type = autofill::ACCOUNT_CREATION_PASSWORD}},
               {.role = ElementRole::NEW_PASSWORD,
                .form_control_type = "password",