Add WorkerPool::PostTaskAndReply and use in DHCP code.

base/threading/worker_pool.h

Index: base/threading/worker_pool.h
diff --git a/base/threading/worker_pool.h b/base/threading/worker_pool.h
index 697cc7c5b18e5cdc62ddcd89027db781833504bc..f8d5ad51ad6405d525ee73935662701902cdaa3b 100644
--- a/base/threading/worker_pool.h
+++ b/base/threading/worker_pool.h
@@ -38,6 +38,14 @@ class BASE_EXPORT WorkerPool {
                        Task* task, bool task_is_slow);
   static bool PostTask(const tracked_objects::Location& from_here,
                        const base::Closure& task, bool task_is_slow);
+
+  // Just like MessageLoopProxy::PostTaskAndReply, except the destination
+  // for |task| is a worker thread and you can specify |task_is_slow| just
+  // like you can for PostTask above.
+  static bool PostTaskAndReply(const tracked_objects::Location& from_here,
+                               const Closure& task,
+                               const Closure& reply,
+                               bool task_is_slow);
 };
 
 }  // namespace base