Auto merge of #11373 - servo:threadfactory, r=larsbergstrom

Use associated types to improve LayoutThreadFactory and ScriptThreadFactory.

Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data:
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy --faster` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

Either:
- [ ] There are tests for these changes OR
- [x] These changes do not require tests because refactoring

Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process.

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components/servo/lib.rs

@@ -231,7 +231,8 @@ fn create_constellation(opts: opts::Opts,
         webrender_api_sender: webrender_api_sender,
     };
     let constellation_chan =
-        Constellation::<layout::layout_thread::LayoutThread,
+        Constellation::<script::layout_interface::Msg,
+                        layout::layout_thread::LayoutThread,
                         script::script_thread::ScriptThread>::start(initial_state);
 
     // Send the URL command to the constellation.
@@ -264,7 +265,8 @@ pub fn run_content_process(token: String) {
 
     script::init();
 
-    unprivileged_content.start_all::<layout::layout_thread::LayoutThread,
+    unprivileged_content.start_all::<script::layout_interface::Msg,
+                                     layout::layout_thread::LayoutThread,
                                      script::script_thread::ScriptThread>(true);
 }