[net/dns] Perform A/AAAA queries for AF_UNSPEC resolutions in parallel.

net/base/prioritized_dispatcher.cc

Index: net/base/prioritized_dispatcher.cc
===================================================================
--- net/base/prioritized_dispatcher.cc	(revision 212872)
+++ net/base/prioritized_dispatcher.cc	(working copy)
@@ -45,6 +45,18 @@
   return queue_.Insert(job, priority);
 }
 
+PrioritizedDispatcher::Handle PrioritizedDispatcher::AddAtHead(
+    Job* job, Priority priority) {
+  DCHECK(job);
+  DCHECK_LT(priority, num_priorities());
+  if (num_running_jobs_ < max_running_jobs_[priority]) {
+    ++num_running_jobs_;
+    job->Start();
+    return Handle();
+  }
+  return queue_.InsertAtFront(job, priority);
+}
+
 void PrioritizedDispatcher::Cancel(const Handle& handle) {
   queue_.Erase(handle);
 }