Run rustfmt on selectors, servo_arc, and style.

This was generated with:

./mach cargo fmt --package selectors &&
./mach cargo fmt --package servo_arc &&
./mach cargo fmt --package style

Using rustfmt 0.4.1-nightly (a4462d1 2018-03-26)

components/style/values/generics/counters.rs

@@ -75,7 +75,7 @@ where
         W: fmt::Write,
     {
         if self.0.is_empty() {
-            return dest.write_str("none")
+            return dest.write_str("none");
         }
 
         let mut first = true;