style: Remove a couple trivial dependencies on nsPresContext.

Differential Revision: https://phabricator.services.mozilla.com/D20141

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

@@ -98,7 +98,7 @@ pub mod system_colors {
             unsafe {
                 Gecko_GetLookAndFeelSystemColor(
                     *self as i32,
-                    cx.device().pres_context(),
+                    cx.device().document(),
                 )
             }
         }

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

@@ -396,7 +396,7 @@ ${helpers.predefined_type(
                         &mut system,
                         id as i32,
                         cx.style().get_font().gecko(),
-                        cx.device().pres_context()
+                        cx.device().document()
                     )
                 }
                 let font_weight = longhands::font_weight::computed_value::T::from_gecko_weight(system.weight);