Add support for priorities in TaskQueue

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..389aeb61b9c2b4ee4f02d1ad69faf77ac6661eaa 100644
--- a/webrtc/base/task_queue_gcd.cc
+++ b/webrtc/base/task_queue_gcd.cc
@@ -21,6 +21,19 @@
 #include "webrtc/base/task_queue_posix.h"
 
 namespace rtc {
+namespace {
+int TaskQueuePriorityToGCD(TaskQueue::Priority priority) {
+  switch (priority) {
+    case TaskQueue::NORMAL:
+      return DISPATCH_QUEUE_PRIORITY_DEFAULT;
+    case TaskQueue::HIGH:
+      return DISPATCH_QUEUE_PRIORITY_HIGH;
+    case TaskQueue::LOW:
+      return DISPATCH_QUEUE_PRIORITY_LOW;
+  }
+}
+}
+
 using internal::GetQueuePtrTls;
 using internal::AutoSetCurrentQueuePtr;
 
@@ -94,7 +107,7 @@ struct TaskQueue::PostTaskAndReplyContext : public TaskQueue::TaskContext {
   std::unique_ptr<QueuedTask> first_task;
 };
 
-TaskQueue::TaskQueue(const char* queue_name)
+TaskQueue::TaskQueue(const char* queue_name, Priority priority /*= NORMAL*/)
     : queue_(dispatch_queue_create(queue_name, DISPATCH_QUEUE_SERIAL)),
       context_(new QueueContext(this)) {
   RTC_DCHECK(queue_name);
@@ -104,6 +117,9 @@ TaskQueue::TaskQueue(const char* queue_name)
   // to the queue is released.  This may run after the TaskQueue object has
   // been deleted.
   dispatch_set_finalizer_f(queue_, &QueueContext::DeleteContext);
+
+  dispatch_set_target_queue(
+      queue_, dispatch_get_global_queue(TaskQueuePriorityToGCD(priority), 0));
 }
 
 TaskQueue::~TaskQueue() {