Bug 1373739 - Apply thumb styles directly to their orientation variants. r=mossop

Previously the min-height style from vertical thumbs was allowed to
cascade down to horizontal thumbs. This caused broken scrollbar sizing
behavior when running without a native (platform/widget toolkit) theme:
horizontal scrollbars would have a non-zero height and width while
vertical scrollbars would have a zero width and non-zero height.
This broke one of the web platform tests in headless mode
(/intersection-observer/root-margin.html).

MozReview-Commit-ID: 4Y3jz77Z0UA

--HG--
extra : rebase_source : 4e21e0e40e1ef9404486a0417a3ec7ebcfd37041

toolkit/themes/osx/global/nativescrollbars.css

@@ -41,14 +41,14 @@ slider[orient="vertical"] {
 
 /* ..... thumb ..... */
 
-thumb {
-  -moz-appearance: scrollbarthumb-horizontal;
-}
-
 thumb[orient="vertical"] {
   -moz-appearance: scrollbarthumb-vertical;
 }
 
+thumb[orient="horizontal"] {
+  -moz-appearance: scrollbarthumb-horizontal;
+}
+
 /* ..... increment ..... */
 
 scrollbarbutton[type="increment"] {

toolkit/themes/windows/global/xulscrollbars.css

@@ -44,7 +44,7 @@ slider[orient="vertical"] {
 
 /* ::::: thumb (horizontal) ::::: */
 
-thumb {
+thumb[orient="vertical"] {
   -moz-appearance: scrollbarthumb-vertical;
   min-height: 8px;
 }