Delete unused line in DnsTransaction.

BUG=None

Change-Id: I1eae52d89a5023ca853ffd3de1f884e174bdd0da
Reviewed-on: https://chromium-review.googlesource.com/1143173Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576472}

net/dns/dns_transaction.cc

@@ -1297,7 +1297,6 @@ class DnsTransactionImpl : public DnsTransaction,
   // Clears and cancels all non-completed attempts. If |leave_attempt| is not
   // null, it is not cleared even if complete.
   void ClearAttempts(const DnsAttempt* leave_attempt) {
-    std::unique_ptr<DnsAttempt> completed_result;
     for (auto it = attempts_.begin(); it != attempts_.end();) {
       if (!(*it)->is_completed() && it->get() != leave_attempt) {
         it = attempts_.erase(it);