Fix crash in ServiceResolverImplMac.

This was a latent bug until Callback was changed to OnceCallback
because callback could be called more than once.

Bug: 778149
Change-Id: I8268de7e61c31406aa20f9005f37c08055bab03d
Reviewed-on: https://chromium-review.googlesource.com/744023Reviewed-by: Gene Gutnik <gene@chromium.org>
Commit-Queue: Vladislav Kuzkokov <vkuzkokov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512621}

chrome/browser/local_discovery/service_discovery_client_mac.mm

@@ -357,6 +357,9 @@ ServiceResolverImplMac::NetServiceContainer::StartResolvingOnDiscoveryThread() {
 
 void ServiceResolverImplMac::NetServiceContainer::OnResolveUpdate(
     RequestStatus status) {
+  if (callback_.is_null())
+    return;
+
   if (status != STATUS_SUCCESS) {
     callback_runner_->PostTask(
         FROM_HERE,
@@ -435,7 +438,8 @@ void ServiceResolverImplMac::OnResolveComplete(
 
   has_resolved_ = true;
 
-  std::move(callback_).Run(status, description);
+  if (!callback_.is_null())
+    std::move(callback_).Run(status, description);
 }
 
 ServiceResolverImplMac::NetServiceContainer*