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

content/worker/worker_thread.cc

Index: content/worker/worker_thread.cc
diff --git a/content/worker/worker_thread.cc b/content/worker/worker_thread.cc
index c15b3063d3d1e7eff84bb4fdbb9215aa3df8116e..916b2192eeee2b5c1304674cc6f0957dec49dcb1 100644
--- a/content/worker/worker_thread.cc
+++ b/content/worker/worker_thread.cc
@@ -58,6 +58,8 @@ WorkerThread::~WorkerThread() {
 }
 
 void WorkerThread::Shutdown() {
+  ChildThread::Shutdown();
+
   // Shutdown in reverse of the initialization order.
   channel()->RemoveFilter(indexed_db_message_filter_.get());
   indexed_db_message_filter_ = NULL;