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

content/child/child_thread.h

Index: content/child/child_thread.h
diff --git a/content/child/child_thread.h b/content/child/child_thread.h
index aa04e2e23cbfd26a057fae0a47b6ce27359364d5..eb060ee1f7b87240874307bf1399d7887d628745 100644
--- a/content/child/child_thread.h
+++ b/content/child/child_thread.h
@@ -50,7 +50,7 @@ class CONTENT_EXPORT ChildThread : public IPC::Listener, public IPC::Sender {
   // has a thread that post tasks to ChildProcess::main_thread(), that thread
   // should be joined in Shutdown().
   virtual ~ChildThread();
-  virtual void Shutdown() = 0;
+  virtual void Shutdown();
 
   // IPC::Sender implementation:
   virtual bool Send(IPC::Message* msg) OVERRIDE;