Fix potential user-after-move in //net/dns/

Fix use-after-move (potential) bug found by the
"bugprone-use-after-move" clang-tidy check.

Bug: 1122844
Change-Id: Ife22d3faf7d10289469367080065ff5cffe93e8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2385359Reviewed-by: Eric Orth <ericorth@chromium.org>
Commit-Queue: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807670}

net/dns/dns_transaction.cc

@@ -961,7 +961,8 @@ class DnsOverHttpsProbeRunner : public DnsProbeRunner {
         &probe_stats->probe_attempts, context_->url_request_context(),
         context_->isolation_info(), RequestPriority::DEFAULT_PRIORITY);
 
-    probe_stats->probe_attempts.back()->Start(base::BindOnce(
+    DnsAttempt* probe_attempt = probe_stats->probe_attempts.back().get();
+    probe_attempt->Start(base::BindOnce(
         &DnsOverHttpsProbeRunner::ProbeComplete, weak_ptr_factory_.GetWeakPtr(),
         attempt_number, doh_server_index, std::move(probe_stats),
         network_change, sequence_start_time,