Import of the hotfix to Firestore release 1.10.0 from GitHub.

The only manual change is to use the newly-added `AsyncQueue::Create` function.

Copybara-generated description follows:

Import of Firestore from GitHub.

  - c2ec9dac574455a040cbb6ad591110a8c3a04f37 Fixes for issues discovered in the Unity desktop build (#... by Gil <mcg@google.com>

PiperOrigin-RevId: 293454352

Author: var-const

firestore/src/ios/firestore_ios.cc

@@ -35,9 +35,9 @@ using util::AsyncQueue;
 using util::Executor;
 using util::Status;
 
-std::unique_ptr<AsyncQueue> CreateWorkerQueue() {
+std::shared_ptr<AsyncQueue> CreateWorkerQueue() {
   auto executor = Executor::CreateSerial("com.google.firebase.firestore");
-  return absl::make_unique<AsyncQueue>(std::move(executor));
+  return AsyncQueue::Create(std::move(executor));
 }
 
 std::unique_ptr<CredentialsProvider> CreateCredentialsProvider(App* app) {