Factor out common StepRange construction code.

Provide an InputType helper method for creating a StepRange given the
default values to use. Use it for all InputTypes that have a step
range bar Range.

R=
BUG=257369

Review URL: https://codereview.chromium.org/108513007

git-svn-id: svn://svn.chromium.org/blink/trunk@163656 bbb929c8-8fbe-4397-9dbb-9b2b20218538