Use mojo SystemTray interfaces for both mash and classic ash

ash/shell.cc

Index: ash/shell.cc
diff --git a/ash/shell.cc b/ash/shell.cc
index 8c92ec3d2bbe7a9d751d20d0a4dbb2fce8be4a3b..9ee9c886b7052a74c804be6328ae6bf26dafcb0e 100644
--- a/ash/shell.cc
+++ b/ash/shell.cc
@@ -607,7 +607,8 @@ Shell::~Shell() {
 }
 
 void Shell::Init(const ShellInitParams& init_params) {
-  wm_shell_->Initialize(init_params.blocking_pool);
+  // HACK: In real life, this would have a shell::Connector as well.
+  wm_shell_->Initialize(init_params.blocking_pool, nullptr);
 
   immersive_handler_factory_ = base::MakeUnique<ImmersiveHandlerFactoryAsh>();