Compatible handling of the form.autocomplete IDL attribute.

Align with the spec and Gecko for the enumerated attribute
'autocomplete' over form elements. That is, map both missing and
invalid content attribute values to the "on" keyword/string.

R=
BUG=333229

Review URL: https://codereview.chromium.org/133703002

git-svn-id: svn://svn.chromium.org/blink/trunk@165043 bbb929c8-8fbe-4397-9dbb-9b2b20218538