Issue #1705 - Part 4: Add scrollbar-width CSS keyword to CSS parser.

This should be all parts needed to add a brand new enum keyword including
getting the computed style from it...

layout/style/nsComputedDOMStyle.cpp

@@ -3305,6 +3305,16 @@ nsComputedDOMStyle::DoGetScrollSnapTypeY()
   return val.forget();
 }
 
+already_AddRefed<CSSValue>
+nsComputedDOMStyle::DoGetScrollbarWidth()
+{
+  RefPtr<nsROCSSPrimitiveValue> val = new nsROCSSPrimitiveValue;
+  val->SetIdent(
+    nsCSSProps::ValueToKeywordEnum(StyleUserInterface()->mScrollbarWidth,
+                                   nsCSSProps::kScrollbarWidthKTable));
+  return val.forget();
+}
+
 already_AddRefed<CSSValue>
 nsComputedDOMStyle::GetScrollSnapPoints(const nsStyleCoord& aCoord)
 {