Update input[type=range]'s step base handling to match spec.

The HTML spec recently shifted to using 'value' as the first fallback
option for an input element's "step base" (if no 'min' attribute),

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

Also bring type=range elements into line with that, including using
that step base when clamping values to the supported range.

R=
BUG=339194

Review URL: https://codereview.chromium.org/152753002

git-svn-id: svn://svn.chromium.org/blink/trunk@166367 bbb929c8-8fbe-4397-9dbb-9b2b20218538