Use last resort font when only there is no font in font group.

There is a little bit of performance gain of layout.

src/components/gfx/font_context.rs

@@ -154,6 +154,7 @@ impl<'self> FontContext {
             }
         }
 
+        if fonts.len() == 0 {
             let last_resort = FontList::get_last_resort_font_families();
 
             for family in last_resort.iter() {
@@ -174,6 +175,7 @@ impl<'self> FontContext {
                     }
                 }
             }
+        }
 
         assert!(fonts.len() > 0);
         // TODO(Issue #179): Split FontStyle into specified and used styles