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

net/base/prioritized_dispatcher.h

Index: net/base/prioritized_dispatcher.h
===================================================================
--- net/base/prioritized_dispatcher.h	(revision 218288)
+++ net/base/prioritized_dispatcher.h	(working copy)
@@ -78,6 +78,10 @@
   // it is queued in the dispatcher.
   Handle Add(Job* job, Priority priority);
 
+  // Just like Add, except that it adds Job at the font of queue of jobs with
+  // priorities of |priority|.
+  Handle AddAtHead(Job* job, Priority priority);
+
   // Removes the job with |handle| from the queue. Invalidates |handle|.
   // Note: a Handle is valid iff the job is in the queue, i.e. has not Started.
   void Cancel(const Handle& handle);