style: Ignore pref-controlled properties in cached scrollbar style assertions.

Now if you add a new inherited, pref-controlled property, you must
declare whether it can have an effect on scrollbar styles.  If no,
then the property will be skipped in the assertions that check
whether our cached styles are equal to those we would compute.

Differential Revision: https://phabricator.services.mozilla.com/D37507

components/style/properties/longhands/inherited_ui.mako.rs

@@ -74,6 +74,10 @@ ${helpers.predefined_type(
     "Default::default()",
     spec="https://drafts.csswg.org/css-scrollbars-1/#scrollbar-color",
     gecko_pref="layout.css.scrollbar-color.enabled",
+    # Surprisingly, yes the computed value of scrollbar-color has no effect on
+    # Gecko scrollbar elements, since the value only matters on the scrollable
+    # element itself.
+    has_effect_on_gecko_scrollbars=False,
     animation_value_type="ScrollbarColor",
     boxed=True,
     ignored_when_colors_disabled=True,