Add Websocket task source

According to the doc: https://html.spec.whatwg.org/multipage/web-sockets.html#network

The task source for all tasks queued in the websocket section are the
websocket task source, so this commit also updates those references to
use the appropriate one.

components/script/task_source/websocket.rs

@@ -0,0 +1,37 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * 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/. */
+
+use msg::constellation_msg::PipelineId;
+use script_runtime::{CommonScriptMsg, ScriptChan, ScriptThreadEventCategory};
+use task::{TaskCanceller, TaskOnce};
+use task_source::{TaskSource, TaskSourceName};
+
+#[derive(JSTraceable)]
+pub struct WebsocketTaskSource(pub Box<ScriptChan + Send + 'static>, pub PipelineId);
+
+impl Clone for WebsocketTaskSource {
+    fn clone(&self) -> WebsocketTaskSource {
+        WebsocketTaskSource(self.0.clone(), self.1.clone())
+    }
+}
+
+impl TaskSource for WebsocketTaskSource {
+    const NAME: TaskSourceName = TaskSourceName::Websocket;
+
+    fn queue_with_canceller<T>(
+        &self,
+        task: T,
+        canceller: &TaskCanceller,
+    ) -> Result<(), ()>
+    where
+        T: TaskOnce + 'static,
+    {
+        self.0.send(CommonScriptMsg::Task(
+            ScriptThreadEventCategory::NetworkEvent,
+            Box::new(canceller.wrap_task(task)),
+            Some(self.1),
+            WebsocketTaskSource::NAME,
+        ))
+    }
+}