script: Make most of 2D canvas and WebGL run over IPC.

To actually make the multiprocess communication work, we'll need to
reroute the task creation to the pipeline or the compositor. But this
works as a first step.

components/script/dom/bindings/global.rs

@@ -17,7 +17,7 @@ use dom::window::{self, WindowHelpers};
 use devtools_traits::DevtoolsControlChan;
 use script_task::{ScriptChan, ScriptPort, ScriptMsg, ScriptTask};
 
-use msg::constellation_msg::{PipelineId, WorkerId};
+use msg::constellation_msg::{ConstellationChan, PipelineId, WorkerId};
 use net_traits::ResourceTask;
 use profile_traits::mem;
 
@@ -91,6 +91,14 @@ impl<'a> GlobalRef<'a> {
         }
     }
 
+    /// Get a `ConstellationChan` to send messages to the constellation channel when available.
+    pub fn constellation_chan(&self) -> ConstellationChan {
+        match *self {
+            GlobalRef::Window(window) => window.constellation_chan(),
+            GlobalRef::Worker(worker) => worker.constellation_chan(),
+        }
+    }
+
     /// Get a `DevtoolsControlChan` to send messages to Devtools
     /// task when available.
     pub fn devtools_chan(&self) -> Option<DevtoolsControlChan> {

components/script/dom/bindings/trace.rs

@@ -62,8 +62,8 @@ use msg::compositor_msg::ScriptListener;
 use msg::constellation_msg::ConstellationChan;
 use net_traits::image::base::Image;
 use profile_traits::mem::ProfilerChan;
-use serde::Serialize;
 use util::str::{LengthOrPercentageOrAuto};
+use serde::{Deserialize, Serialize};
 use std::cell::{Cell, UnsafeCell, RefCell};
 use std::collections::{HashMap, HashSet};
 use std::collections::hash_state::HashState;
@@ -332,6 +332,13 @@ impl<A,B> JSTraceable for fn(A) -> B {
     }
 }
 
+impl<T> JSTraceable for IpcSender<T> where T: Deserialize + Serialize {
+    #[inline]
+    fn trace(&self, _: *mut JSTracer) {
+        // Do nothing
+    }
+}
+
 impl JSTraceable for ScriptListener {
     #[inline]
     fn trace(&self, _: *mut JSTracer) {
@@ -346,13 +353,6 @@ impl JSTraceable for Box<LayoutRPC+'static> {
     }
 }
 
-impl<T> JSTraceable for IpcSender<T> where T: Serialize {
-    #[inline]
-    fn trace(&self, _: *mut JSTracer) {
-        // Do nothing
-    }
-}
-
 impl JSTraceable for () {
     #[inline]
     fn trace(&self, _trc: *mut JSTracer) {