Adds PriorityQueue and PrioritizedDispatcher.

net/dns/dns_transaction.h

Index: net/dns/dns_transaction.h
diff --git a/net/dns/dns_transaction.h b/net/dns/dns_transaction.h
index e6a1d029d934211afcb455c80f021b4d393679e2..afaabec0b45a7afec5bcaeb349197bcd65c008e3 100644
--- a/net/dns/dns_transaction.h
+++ b/net/dns/dns_transaction.h
@@ -49,8 +49,9 @@ class NET_EXPORT_PRIVATE DnsTransaction :
 
   const DnsResponse* response() const { return response_.get(); }
 
-  // Starts the resolution process.  Will return ERR_IO_PENDING and will
-  // notify the caller via |delegate|.  Should only be called once.
+  // Starts the resolution process. Will return ERR_IO_PENDING and will notify
+  // the caller via |delegate|, except when failing synchronously. Should only
+  // be called once.
   int Start();
 
  private:
@@ -64,6 +65,12 @@ class NET_EXPORT_PRIVATE DnsTransaction :
     STATE_NONE,
   };
 
+  struct Attempt {
+    scoped_ptr<DnsQuery> query_;
+    scoped_ptr<DnsResponse> response_;
+    scoped_ptr<DatagramClientSocket> socket_;
+  };
+
   int DoLoop(int result);
   void DoCallback(int result);
   void OnIOComplete(int result);