Fix potential deadlock in TaskQueue's libevent PostTaskAndReply implementation

webrtc/base/task_queue_gcd.cc

Index: webrtc/base/task_queue_gcd.cc
diff --git a/webrtc/base/task_queue_gcd.cc b/webrtc/base/task_queue_gcd.cc
index 2c7d649fc9598723994189ee39f82b26e89c1111..ea0bdc970e54f5a28a8309495e9d5fa323bc09ef 100644
--- a/webrtc/base/task_queue_gcd.cc
+++ b/webrtc/base/task_queue_gcd.cc
@@ -69,13 +69,14 @@ struct TaskQueue::PostTaskAndReplyContext : public TaskQueue::TaskContext {
                                    std::unique_ptr<QueuedTask> second_task)
       : TaskContext(second_queue_ctx, std::move(second_task)),
         first_queue_ctx(first_queue_ctx),
-        first_task(std::move(first_task)) {
+        first_task(std::move(first_task)),
+        reply_queue_(second_queue_ctx->queue->queue_) {
     // Retain the reply queue for as long as this object lives.
     // If we don't, we may have memory leaks and/or failures.
-    dispatch_retain(first_queue_ctx->queue->queue_);
+    dispatch_retain(reply_queue_);
   }
   ~PostTaskAndReplyContext() override {
-    dispatch_release(first_queue_ctx->queue->queue_);
+    dispatch_release(reply_queue_);
   }
 
   static void RunTask(void* context) {
@@ -87,11 +88,12 @@ struct TaskQueue::PostTaskAndReplyContext : public TaskQueue::TaskContext {
     }
     // Post the reply task.  This hands the work over to the parent struct.
     // This task will eventually delete |this|.
-    dispatch_async_f(rc->queue_ctx->queue->queue_, rc, &TaskContext::RunTask);
+    dispatch_async_f(rc->reply_queue_, rc, &TaskContext::RunTask);
   }
 
   QueueContext* const first_queue_ctx;
   std::unique_ptr<QueuedTask> first_task;
+  dispatch_queue_t reply_queue_;
 };
 
 TaskQueue::TaskQueue(const char* queue_name)