services/ui: Have an explicit lifetime/ownership of ui::GpuService.

services/ui/demo/mus_demo.cc

Index: services/ui/demo/mus_demo.cc
diff --git a/services/ui/demo/mus_demo.cc b/services/ui/demo/mus_demo.cc
index 89b601e034eb74f060cbdab7a9c96437cd90e7cc..3710de8d30fced9718230035536fb8a988fd0608 100644
--- a/services/ui/demo/mus_demo.cc
+++ b/services/ui/demo/mus_demo.cc
@@ -65,7 +65,7 @@ MusDemo::~MusDemo() {
 }
 
 void MusDemo::OnStart(const shell::Identity& identity) {
-  GpuService::Initialize(connector());
+  gpu_service_ = GpuService::Initialize(connector());
   window_tree_client_ = new WindowTreeClient(this, this, nullptr);
   window_tree_client_->ConnectAsWindowManager(connector());
 }