mus::GpuService: Support establish GpuChannel asynchronously.

mash/app_driver/app_driver.cc

Index: mash/app_driver/app_driver.cc
diff --git a/mash/app_driver/app_driver.cc b/mash/app_driver/app_driver.cc
index 24d4651f3868c513aa5dd9ad904acccedab5d97f..0c87bcf2048426c910669f4f5c913460f5e1b55a 100644
--- a/mash/app_driver/app_driver.cc
+++ b/mash/app_driver/app_driver.cc
@@ -9,6 +9,7 @@
 #include "base/bind.h"
 #include "base/message_loop/message_loop.h"
 #include "components/mus/common/event_matcher_util.h"
+#include "components/mus/common/gpu_service.h"
 #include "mash/public/interfaces/launchable.mojom.h"
 #include "services/shell/public/cpp/connection.h"
 #include "services/shell/public/cpp/connector.h"
@@ -89,6 +90,7 @@ void AppDriver::Initialize(shell::Connector* connector,
                            const shell::Identity& identity,
                            uint32_t id) {
   connector_ = connector;
+  mus::GpuService::Initialize(connector);
   AddAccelerators();
 }