Implement ChildThread::shutdown and clear members that have refs into blink

content/plugin/plugin_thread.cc

Index: content/plugin/plugin_thread.cc
diff --git a/content/plugin/plugin_thread.cc b/content/plugin/plugin_thread.cc
index ec74d2e01f1d53a00ce5635e7eefdad807b28af8..5faeb74468d84442af5d88bc9c089d6eed34a0d3 100644
--- a/content/plugin/plugin_thread.cc
+++ b/content/plugin/plugin_thread.cc
@@ -139,6 +139,8 @@ void PluginThread::SetForcefullyTerminatePluginProcess() {
 }
 
 void PluginThread::Shutdown() {
+  ChildThread::Shutdown();
+
   if (preloaded_plugin_module_) {
     base::UnloadNativeLibrary(preloaded_plugin_module_);
     preloaded_plugin_module_ = NULL;