Derive the step base for an input element as (now) specified.

Determine the step base for an input element per spec,

  http://www.whatwg.org/specs/web-apps/current-work/#concept-input-min-zero

That is, consult the 'value' attribute if 'min' is not present, and
then fallback to the default step base for the input type if that
isn't present either. Previously, 'value' was not considered.

R=
BUG=257369

Review URL: https://codereview.chromium.org/83413002

git-svn-id: svn://svn.chromium.org/blink/trunk@162733 bbb929c8-8fbe-4397-9dbb-9b2b20218538