Bug 1008197. We need to always have the overlay styles on scrollbars on mac because whether we have overlay scrollbars changes dynamically. r=roc

This backs out the mac part of bug 994861. Windows doesn't change overlay scrollbars dynamically, so the Windows part of bug 994861 can stay.

toolkit/themes/osx/global/nativescrollbars.css

@@ -13,16 +13,9 @@ scrollbar {
   pointer-events: auto;
 }
 
-@media all and (-moz-overlay-scrollbars) {
-  scrollbar[root="true"] {
-    position: relative;
-    z-index: 2147483647; /* largest positive value of a signed 32-bit integer */
-  }
-
-  scrollbar:not([active="true"]),
-  scrollbar[disabled="true"] {
-    visibility: hidden;
-  }
+scrollbar[root="true"] {
+  position: relative;
+  z-index: 2147483647; /* largest positive value of a signed 32-bit integer */
 }
 
 html|select[size]:not([size="0"]):not([size="1"]) > scrollbar,
@@ -30,6 +23,13 @@ html|select[multiple] > scrollbar {
   -moz-appearance: scrollbar-small;
 }
 
+@media all and (-moz-overlay-scrollbars) {
+  scrollbar:not([active="true"]),
+  scrollbar[disabled="true"] {
+    visibility: hidden;
+  }
+}
+
 /* ..... track ..... */
 
 slider {