Instant Tethering: Replace {} with default in destructor.

I missed this change in crrev.com/c/1343280.

R=hansberry@chromium.org

Bug: none
Test: none
Change-Id: I57d525bc41c883e86812ca4a2d09faf018b9210d
Reviewed-on: https://chromium-review.googlesource.com/c/1357654Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Commit-Queue: James Hawkins <jhawkins@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612897}

chromeos/components/tether/asynchronous_shutdown_object_container_impl.cc

@@ -88,8 +88,7 @@ AsynchronousShutdownObjectContainerImpl::
           network_configuration_remover_.get())) {}
 
 AsynchronousShutdownObjectContainerImpl::
-    ~AsynchronousShutdownObjectContainerImpl() {
-}
+    ~AsynchronousShutdownObjectContainerImpl() = default;
 
 void AsynchronousShutdownObjectContainerImpl::Shutdown(
     const base::Closure& shutdown_complete_callback) {