Bug 1456518 - Shut down the gray mark observers on worker threads, r=jonco

--HG--
extra : topic : gray.crash
extra : rebase_source : 90a65d8f6c77357d8fef1bcae929179275bd0b5f

js/src/shell/js.cpp

@@ -3576,6 +3576,7 @@ WorkerMain(void* arg)
 
     auto guard = mozilla::MakeScopeExit([&] {
         CancelOffThreadJobsForContext(cx);
+        sc->markObservers.reset();
         JS_DestroyContext(cx);
         js_delete(sc);
         js_delete(input);

js/src/tests/non262/regress/regress-1456518-workergray.js

@@ -0,0 +1,7 @@
+if (typeof 'evalInWorder' == 'function') {
+    evalInWorker(`
+  addMarkObservers([grayRoot(), grayRoot().x, this, Object.create(null)]);
+`);
+}
+
+reportCompare('do not crash', 'do not crash', 'did not crash!');