Remove the inherited_style getter from StyleBuilder.

The concept of inherited style is about to get a bit more complicated, and this
will prevent consumers from doing it wrong.

Part 1 of Gecko bug1382806.  r=emilio

components/style/values/specified/length.rs

@@ -93,7 +93,7 @@ impl FontBaseSize {
         match *self {
             FontBaseSize::Custom(size) => size,
             FontBaseSize::CurrentStyle => context.style().get_font().clone_font_size(),
-            FontBaseSize::InheritedStyle => context.inherited_style().get_font().clone_font_size(),
+            FontBaseSize::InheritedStyle => context.style().get_parent_font().clone_font_size(),
         }
     }
 }