[css-scrollbars-1] Add scrollbar-width: none.

Author: upsuper

css-scrollbars-1/Overview.bs

@@ -127,7 +127,7 @@ See related <a href="https://github.com/w3c/csswg-drafts/issues/1956">Issue 1956
 
 <pre class="propdef"> 
 Name: scrollbar-width 
-Value: auto | thin | <<length>>
+Value: auto | thin | none | <<length>>
 Initial: auto
 Inherited: no
 Computed value: absolute length or the keyword auto.
@@ -146,6 +146,9 @@ This property allows the author to set the maximum thickness of an element’s s
 <dd>implementations must use either a thin scrollbar width variant on platforms that provide that option, 
 or display a scrollbar thinner than the default platform scrollbar width.
 </dd>
+<dt><dfn>none</dfn></dt>
+<dd>implementations must not display any scrollbar, however the element's scrollability is not affected.
+</dd>
 <dt><dfn><<length>></dfn></dt>
 <dd>When a user agent displays one or more scrollbars for an element, 
 the thickness of those scrollbars must be no more than the computed 'scrollbar-width' length value.