Remove use of unstable box syntax.

http://www.robohornet.org gives a score of 101.36 on master,
and 102.68 with this PR. The latter is slightly better,
but probably within noise level.
So it looks like this PR does not affect DOM performance.

This is expected since `Box::new` is defined as:

```rust
impl<T> Box<T> {
    #[inline(always)]
    pub fn new(x: T) -> Box<T> {
        box x
    }
}
```

With inlining, it should compile to the same as box syntax.

components/script/task_source/dom_manipulation.rs

@@ -36,7 +36,7 @@ impl TaskSource for DOMManipulationTaskSource {
     {
         let msg = MainThreadScriptMsg::Common(CommonScriptMsg::Task(
             ScriptThreadEventCategory::ScriptEvent,
-            box canceller.wrap_task(task),
+            Box::new(canceller.wrap_task(task)),
         ));
         self.0.send(msg).map_err(|_| ())
     }

components/script/task_source/file_reading.rs

@@ -29,7 +29,7 @@ impl TaskSource for FileReadingTaskSource {
     {
         self.0.send(CommonScriptMsg::Task(
             ScriptThreadEventCategory::FileRead,
-            box canceller.wrap_task(task),
+            Box::new(canceller.wrap_task(task)),
         ))
     }
 }

components/script/task_source/history_traversal.rs

@@ -15,6 +15,6 @@ impl ScriptChan for HistoryTraversalTaskSource {
     }
 
     fn clone(&self) -> Box<ScriptChan + Send> {
-        box HistoryTraversalTaskSource((&self.0).clone())
+        Box::new(HistoryTraversalTaskSource((&self.0).clone()))
     }
 }

components/script/task_source/networking.rs

@@ -26,7 +26,7 @@ impl TaskSource for NetworkingTaskSource {
     {
         self.0.send(CommonScriptMsg::Task(
             ScriptThreadEventCategory::NetworkEvent,
-            box canceller.wrap_task(task),
+            Box::new(canceller.wrap_task(task)),
         ))
     }
 }
@@ -40,7 +40,7 @@ impl NetworkingTaskSource {
     {
         self.0.send(CommonScriptMsg::Task(
             ScriptThreadEventCategory::NetworkEvent,
-            box task,
+            Box::new(task),
         ))
     }
 }

components/script/task_source/performance_timeline.rs

@@ -40,7 +40,7 @@ impl TaskSource for PerformanceTimelineTaskSource {
     {
         let msg = CommonScriptMsg::Task(
             ScriptThreadEventCategory::PerformanceTimelineTask,
-            box canceller.wrap_task(task)
+            Box::new(canceller.wrap_task(task))
         );
         self.0.send(msg).map_err(|_| ())
     }

components/script/task_source/user_interaction.rs

@@ -36,7 +36,7 @@ impl TaskSource for UserInteractionTaskSource {
     {
         let msg = MainThreadScriptMsg::Common(CommonScriptMsg::Task(
             ScriptThreadEventCategory::InputEvent,
-            box canceller.wrap_task(task),
+            Box::new(canceller.wrap_task(task)),
         ));
         self.0.send(msg).map_err(|_| ())
     }