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/specified/column.rs

@@ -21,6 +21,9 @@ impl Parse for ColumnCount {
         if input.try(|i| i.expect_ident_matching("auto")).is_ok() {
             return Ok(GenericColumnCount::Auto);
         }
-        Ok(GenericColumnCount::Integer(PositiveInteger::parse(context, input)?))
+        Ok(GenericColumnCount::Integer(PositiveInteger::parse(
+            context,
+            input,
+        )?))
     }
 }