Replace compile flag with runtime check for text-underline-position

Replaces the CSS3_TEXT compile flag (from WebKit feature flags) with
runtime check for text-underline-position usage. Also removed the
-webkit- vendor prefix from the CSS property (including layout test
updates).

BUG=239225

Review URL: https://codereview.chromium.org/40733004

git-svn-id: svn://svn.chromium.org/blink/trunk@161235 bbb929c8-8fbe-4397-9dbb-9b2b20218538