WebPasswordFormData: fallback to element's ID if there is no name

In assemblePasswordFormResult, the names for the username, password and old password fields in WebPasswordFormData have been generated from the name attribute.

This CL adds a fallback to the id attribute if name is empty. This is for the password manager to recognise as valid password forms where input elements lack the name attribute but have the id attribute.

Browser test for this is in https://codereview.chromium.org/70273002.
BUG=282487

Review URL: https://codereview.chromium.org/69293008

git-svn-id: svn://svn.chromium.org/blink/trunk@161819 bbb929c8-8fbe-4397-9dbb-9b2b20218538