Fix crash in HTMLInputElement::setupDateTimeChooserParameters

The date time input element has a custom style, meaning that computedStyle will not work here. So delegate the decision, so the input type can do the right thing.

BUG=384591

Review URL: https://codereview.chromium.org/419053005

git-svn-id: svn://svn.chromium.org/blink/trunk@179411 bbb929c8-8fbe-4397-9dbb-9b2b20218538