Auto merge of #11337 - Ms2ger:compositor-exit-wait, r=pcwalton

Don't block the script listener thread on window.close().

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 no tangible effect

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

This does not appear to have any effect. I believe this has been the case
since e841065351, when the helper thread was
introduced.

It was added in a7ef1cd35e, where it blocked the
script thread rather than this helper thread.

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components/compositing/compositor.rs

@@ -528,9 +528,8 @@ impl<Window: WindowMethods> IOCompositor<Window> {
                 return false
             }
 
-            (Msg::Exit(channel), _) => {
+            (Msg::Exit, _) => {
                 self.start_shutting_down();
-                let _ = channel.send(());
             }
 
             (Msg::ShutdownComplete, _) => {