Add ChildMessageFilter, a base class for renderer/worker cross-thread MessageFilter

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 eb6df3bd3349b8faa5eafefae5f6a4c35ebb9b62..54764d0229ce8bc6253ff53cd20ab19d5e411f73 100644
--- a/content/worker/worker_thread.cc
+++ b/content/worker/worker_thread.cc
@@ -56,7 +56,7 @@ WorkerThread::WorkerThread() {
 
   indexed_db_message_filter_ = new IndexedDBMessageFilter(
       thread_safe_sender());
-  channel()->AddFilter(indexed_db_message_filter_.get());
+  channel()->AddFilter(indexed_db_message_filter_->GetFilter());
 
 }
 
@@ -79,7 +79,6 @@ void WorkerThread::Shutdown() {
   }
 
   // Shutdown in reverse of the initialization order.
-  channel()->RemoveFilter(indexed_db_message_filter_.get());
   indexed_db_message_filter_ = NULL;
 
   channel()->RemoveFilter(db_message_filter_.get());