Add support for priorities to TaskQueue.

webrtc/base/task_queue_libevent.cc

Index: webrtc/base/task_queue_libevent.cc
diff --git a/webrtc/base/task_queue_libevent.cc b/webrtc/base/task_queue_libevent.cc
index c6ce5b3baf3e301550308d407a4866dfd05f8abb..9a2f9e8b285b1ed4559b8a5ab165931631c3c975 100644
--- a/webrtc/base/task_queue_libevent.cc
+++ b/webrtc/base/task_queue_libevent.cc
@@ -84,6 +84,19 @@ void EventAssign(struct event* ev,
   RTC_CHECK_EQ(0, event_base_set(base, ev));
 #endif
 }
+
+ThreadPriority TaskQueuePriorityToThreadPriority(TaskQueue::Priority priority) {
+  switch (priority) {
+    case TaskQueue::HIGH:
+      return kRealtimePriority;
+    case TaskQueue::LOW:
+      return kLowPriority;
+    case TaskQueue::NORMAL:
+    default:

[the sun, 2017/02/24 09:03:43]

If compiler won't warn about an unhandled "case", you should instead of silently
resorting to default:

...
case TaskQueue::NORMAL:
  return kNormalPriority;
default:
  RTC_NOTREACHED();
  return kNormalPriority;
...

[tommi, 2017/02/24 17:34:41]

Done.

+      break;
+  }
+  return kNormalPriority;
+}
 }  // namespace
 
 struct TaskQueue::QueueContext {
@@ -201,10 +214,13 @@ class TaskQueue::SetTimerTask : public QueuedTask {
   const uint32_t posted_;
 };
 
-TaskQueue::TaskQueue(const char* queue_name)
+TaskQueue::TaskQueue(const char* queue_name, Priority priority /*= NORMAL*/)
     : event_base_(event_base_new()),
       wakeup_event_(new event()),
-      thread_(&TaskQueue::ThreadMain, this, queue_name) {
+      thread_(&TaskQueue::ThreadMain,
+              this,
+              queue_name,
+              TaskQueuePriorityToThreadPriority(priority)) {
   RTC_DCHECK(queue_name);
   int fds[2];
   RTC_CHECK(pipe(fds) == 0);