cros: Fix LoginDisplayHost double deletion

We could have two DeleteSoon scheduled at the same time and cause
double deletion. Change LoginDisplayHostMojo::OnFinalize to use
ShutdownDisplayHost() so that they will be consolidated into one.

Bug: 900615
Change-Id: I2f2db9369079adc7d4cf24bb9ef65b15ebc79320
Reviewed-on: https://chromium-review.googlesource.com/c/1313740Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604942}

chrome/browser/chromeos/login/ui/login_display_host_mojo.cc

@@ -150,7 +150,7 @@ void LoginDisplayHostMojo::OnFinalize() {
   if (dialog_)
     dialog_->Close();
 
-  base::ThreadTaskRunnerHandle::Get()->DeleteSoon(FROM_HERE, this);
+  ShutdownDisplayHost();
 }
 
 void LoginDisplayHostMojo::SetStatusAreaVisible(bool visible) {