Auto merge of #11368 - izgzhen:fix-filemanager-exit, r=asajeffrey

Fix FileManager thread panic and other misc improvements

- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy --faster` does not report any errors
- [x] There are tests for these changes

Changes:

- Add shut down logic for FileManager thread
- Add an unit test for filemanager_thread

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components/script/dom/file.rs

@@ -51,8 +51,6 @@ impl File {
     pub fn new_from_selected(window: &Window, selected: SelectedFile) -> Root<File> {
         let name = DOMString::from(selected.filename.to_str().expect("File name encoding error"));
 
-        // FIXME: fix this after PR #11221 is landed
-        let id = selected.id;
         let slice = DataSlice::empty();
 
         let global = GlobalRef::Window(window);