Use box syntax instead of Box::new()

Closes #5417

components/profile/lib.rs

@@ -2,6 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+#![feature(box_syntax)]
 #![feature(collections)]
 #![feature(core)]
 #![cfg_attr(target_os="linux", feature(io))]

components/profile/mem.rs

@@ -118,7 +118,7 @@ impl Profiler {
         // Register the system memory reporter, which will run on the memory profiler's own thread.
         // It never needs to be unregistered, because as long as the memory profiler is running the
         // system memory reporter can make measurements.
-        let system_reporter = Box::new(SystemReporter);
+        let system_reporter = box SystemReporter;
         mem_profiler_chan.send(ProfilerMsg::RegisterReporter("system".to_owned(), system_reporter));
 
         mem_profiler_chan