Call ChildThread::Shutdown in PpapiThread::Shutdown.

BUG=None

Review URL: https://codereview.chromium.org/536783003

Cr-Commit-Position: refs/heads/master@{#293260}

content/ppapi_plugin/ppapi_thread.cc

@@ -125,6 +125,8 @@ PpapiThread::~PpapiThread() {
 }
 
 void PpapiThread::Shutdown() {
+  ChildThread::Shutdown();
+
   ppapi::proxy::PluginGlobals::Get()->set_plugin_proxy_delegate(NULL);
   if (plugin_entry_points_.shutdown_module)
     plugin_entry_points_.shutdown_module();