Revert of Move content's shell connections to the IO thread

content/public/test/mock_render_process_host.cc

Index: content/public/test/mock_render_process_host.cc
diff --git a/content/public/test/mock_render_process_host.cc b/content/public/test/mock_render_process_host.cc
index 5924b5170b3d1df51b16192594dc451f09b81836..a75a3f921a34ad5d40c019d6afd4d146a101117b 100644
--- a/content/public/test/mock_render_process_host.cc
+++ b/content/public/test/mock_render_process_host.cc
@@ -269,6 +269,10 @@
   return remote_interfaces_.get();
 }
 
+shell::Connection* MockRenderProcessHost::GetChildConnection() {
+  return nullptr;
+}
+
 std::unique_ptr<base::SharedPersistentMemoryAllocator>
 MockRenderProcessHost::TakeMetricsAllocator() {
   return nullptr;