Use mojo SystemTray interfaces for both mash and classic ash

ash/shell/shell_delegate_impl.cc

Index: ash/shell/shell_delegate_impl.cc
diff --git a/ash/shell/shell_delegate_impl.cc b/ash/shell/shell_delegate_impl.cc
index 9909d67d9417b831afa246cbf23c9a8f36e989c9..046458fd42b969feebcb0431298cade94bf04cdb 100644
--- a/ash/shell/shell_delegate_impl.cc
+++ b/ash/shell/shell_delegate_impl.cc
@@ -196,6 +196,10 @@ ShellDelegateImpl::ShellDelegateImpl()
 
 ShellDelegateImpl::~ShellDelegateImpl() {}
 
+::shell::Connector* ShellDelegateImpl::GetShellConnector() const {
+  return nullptr;
+}
+
 bool ShellDelegateImpl::IsFirstRunAfterBoot() const {
   return false;
 }