services/ui: Allow injecting the IO thread task runner for gpu channel.

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 b57f02f17041b52d8d7024434efb4d088eae748c..3d2dac1edf7b6363792eeadcf78d329fbb99edde 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) {
-  gpu_service_ = GpuService::Initialize(connector());
+  gpu_service_ = GpuService::Create(connector());
   window_tree_client_ = new WindowTreeClient(this, this, nullptr);
   window_tree_client_->ConnectAsWindowManager(connector());
 }