[C++] Provide a default executor with settings on iOS

PiperOrigin-RevId: 314233757

Author: wu-hui

firestore/src/common/settings.cc

@@ -17,7 +17,9 @@ const char kDefaultHost[] = "firestore.googleapis.com";
 
 }
 
+#if !defined(__APPLE__)
 Settings::Settings() : host_(kDefaultHost) {}
+#endif
 
 void Settings::set_host(std::string host) { host_ = firebase::Move(host); }
 

firestore/src/common/settings_ios.mm

@@ -8,9 +8,20 @@
 namespace firebase {
 namespace firestore {
 
+namespace {
+
+const char kDefaultHost[] = "firestore.googleapis.com";
+
+}
+
 using util::Executor;
 using util::ExecutorLibdispatch;
 
+Settings::Settings()
+    : host_(kDefaultHost),
+      executor_(absl::make_unique<ExecutorLibdispatch>(dispatch_queue_create(
+          "com.google.firebase.firestore.callback", DISPATCH_QUEUE_SERIAL))) {}
+
 std::unique_ptr<Executor> Settings::CreateExecutor() const {
   return absl::make_unique<ExecutorLibdispatch>(dispatch_queue());
 }