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

content/renderer/render_thread_impl.cc

Index: content/renderer/render_thread_impl.cc
diff --git a/content/renderer/render_thread_impl.cc b/content/renderer/render_thread_impl.cc
index 3ce5a94b55756185473fbf4dcbc7f80257b1f6e0..ff361b28161452e72339f12d7b5dcf4c87a910d3 100644
--- a/content/renderer/render_thread_impl.cc
+++ b/content/renderer/render_thread_impl.cc
@@ -433,6 +433,8 @@ void RenderThreadImpl::Shutdown() {
   FOR_EACH_OBSERVER(
       RenderProcessObserver, observers_, OnRenderProcessShutdown());
 
+  ChildThread::Shutdown();
+
   // Wait for all databases to be closed.
   if (web_database_observer_impl_)
     web_database_observer_impl_->WaitForAllDatabasesToClose();