Converts mash to use Shell

ash/mus/shell_delegate_mus.cc

Index: ash/mus/shell_delegate_mus.cc
diff --git a/ash/mus/shell_delegate_mus.cc b/ash/mus/shell_delegate_mus.cc
index 41b69b7b9729a556184736fc9570b35014ad4033..053e0cb64aa2db38120eec4edb76b3d8c6852c7c 100644
--- a/ash/mus/shell_delegate_mus.cc
+++ b/ash/mus/shell_delegate_mus.cc
@@ -84,9 +84,11 @@ class SessionStateDelegateStub : public SessionStateDelegate {
 
 }  // namespace
 
-ShellDelegateMus::ShellDelegateMus(service_manager::Connector* connector)
-    : connector_(connector) {
-  // |connector_| may be null in tests.
+ShellDelegateMus::ShellDelegateMus(
+    service_manager::Connector* connector,
+    std::unique_ptr<SystemTrayDelegate> system_tray_delegate_for_test)
+    : connector_(connector),
+      system_tray_delegate_for_test_(std::move(system_tray_delegate_for_test)) {
 }
 
 ShellDelegateMus::~ShellDelegateMus() {}
@@ -146,6 +148,8 @@ ShelfDelegate* ShellDelegateMus::CreateShelfDelegate(ShelfModel* model) {
 }
 
 SystemTrayDelegate* ShellDelegateMus::CreateSystemTrayDelegate() {
+  if (system_tray_delegate_for_test_)
+    return system_tray_delegate_for_test_.release();
   return new SystemTrayDelegateMus();
 }