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/nsStyleConsts.h

@@ -187,6 +187,13 @@ enum class StyleFloatEdge : uint8_t {
   MarginBox,
 };
 
+// scrollbar-width
+enum class StyleScrollbarWidth : uint8_t {
+  Auto,
+  Thin,
+  None,
+};
+
 // shape-box for shape-outside
 enum class StyleShapeOutsideShapeBox : uint8_t {
   NoBox,