Fix DCHECK(!config_watcher_) in HostProcess::~HostProcess().

When shutting down HostProcess config_watcher_ is supposed to be deleted
on the nework thread. Previously ShutdownOnNetworkThread() was deleting
it, but that line was mistakenly removed in crrev.com/302162 . Adding
it back.

Review URL: https://codereview.chromium.org/885693003

Cr-Commit-Position: refs/heads/master@{#313623}

remoting/host/remoting_me2me_host.cc

@@ -1447,6 +1447,8 @@ void HostProcess::ShutdownOnNetworkThread() {
     shutdown_watchdog_->SetExitCode(*exit_code_out_);
     shutdown_watchdog_->Arm();
 
+    config_watcher_.reset();
+
     if (policy_watcher_.get()) {
       policy_watcher_->StopWatching(
           base::Bind(&HostProcess::OnPolicyWatcherShutdown, this));