[WebVTT] Support text-decoration-{text,color,style} on CSS ::cue pseudo-element

The CSS ::cue pseudo-element is defined on WebVTT specification as
part of the HTML5 <track> element specification. This issue intends to
incrementally add support for the following properties:

- text-decoration-line
- text-decoration-style
- text-decoration-color

BUG=260993

Review URL: https://chromiumcodereview.appspot.com/19516002

git-svn-id: svn://svn.chromium.org/blink/trunk@154500 bbb929c8-8fbe-4397-9dbb-9b2b20218538