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

content/child/child_thread.cc

Index: content/child/child_thread.cc
diff --git a/content/child/child_thread.cc b/content/child/child_thread.cc
index 859d2b2643e4463151504f0e1990e00f4fa880b1..05650ade06f688cd69040d437d44b6c508cdb013 100644
--- a/content/child/child_thread.cc
+++ b/content/child/child_thread.cc
@@ -195,6 +195,12 @@ ChildThread::~ChildThread() {
   g_lazy_tls.Pointer()->Set(NULL);
 }
 
+void ChildThread::Shutdown() {
+  // Delete objects that hold references to blink so derived classes can
+  // safely shutdown blink in their Shutdown implementation.
+  file_system_dispatcher_.reset();
+}
+
 void ChildThread::OnChannelConnected(int32 peer_pid) {
   channel_connected_factory_.InvalidateWeakPtrs();
 }