Fix DBus threading issue in PepperTCPSocketMessageFilter dtor

PepperTCPSocketMessageFilter could be released on IO thread
with an outstanding |firewall_hole_| and indirectly calls
dbus code on IO thread. The CL makes sure |firewall_hole_|
is deleted on UI thread.

Bug: 989072
Change-Id: I9266c21a96fa5b931f81d25bc6c8e91293a5e9a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1726235Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682399}

content/browser/renderer_host/pepper/pepper_tcp_socket_message_filter.cc

@@ -124,6 +124,13 @@ PepperTCPSocketMessageFilter::~PepperTCPSocketMessageFilter() {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
   if (host_)
     host_->RemoveInstanceObserver(instance_, this);
+#if defined(OS_CHROMEOS)
+  // Close the firewall hole on UI thread if there is one.
+  if (firewall_hole_) {
+    BrowserThread::DeleteSoon(BrowserThread::UI, FROM_HERE,
+                              std::move(firewall_hole_));
+  }
+#endif  // defined(OS_CHROMEOS)
   --g_num_tcp_filter_instances;
 }