Coverity report:

** CID 1700566:       Concurrent data access violations  (MISSING_LOCK)
/dom/script/ScriptLoader.cpp: 2620           in mozilla::dom::WasmCompileTask::StealResult(JSContext *, JS::MutableHandle<JSObject *>)()

2614       return TaskResult::Complete;
2615     }
2616
2617     bool WasmCompileTask::StealResult(JSContext* aCx,
2618                                       JS::MutableHandle<JSObject*> aModuleOut) {
2619       JS::Rooted<JSObject*> wasmModuleObject(aCx);
>>>     CID 1700566:         Concurrent data access violations  (MISSING_LOCK)
>>>     Accessing "this->mCompileResult" without holding lock "mozilla::detail::MutexImpl.mMutex". Elsewhere, "mozilla::dom::WasmCompileTask.mCompileResult" is written to with "MutexImpl.mMutex" held 1 out of 1 times.
2620       if (!JS::FinishCompileForESM(aCx, *mCompileArgs, mCompileResult,
2621                                    &wasmModuleObject)) {
2622         return false;
2623       }
2624
2625       aModuleOut.set(JS::CreateWasmSourcePhaseModule(aCx, wasmModuleObject));
```)

mCompileResult is written by Run() with mMutex held, and read here without it.
The read happens on the main thread once the task has completed, so there is
no actual race, but taking the lock is uncontended here and silences Coverity
(CID 1700566).

https://hg.mozilla.org/mozilla-central/rev/904341b854da