aura mus: Install a DefautlCaptureClient in the TestWM.

services/ui/test_wm/test_wm.cc

Index: services/ui/test_wm/test_wm.cc
diff --git a/services/ui/test_wm/test_wm.cc b/services/ui/test_wm/test_wm.cc
index 8d2463aac820bf759484921f3f787287ada5eb1d..d9e2c9eb3bc1aedadccae32d17826513c3c1612a 100644
--- a/services/ui/test_wm/test_wm.cc
+++ b/services/ui/test_wm/test_wm.cc
@@ -13,6 +13,7 @@
 #include "services/service_manager/public/cpp/service.h"
 #include "services/service_manager/public/cpp/service_context.h"
 #include "services/service_manager/public/cpp/service_runner.h"
+#include "ui/aura/client/default_capture_client.h"
 #include "ui/aura/env.h"
 #include "ui/aura/mus/property_converter.h"
 #include "ui/aura/mus/property_utils.h"
@@ -131,6 +132,9 @@ class TestWM : public service_manager::Service,
     DCHECK(!root_);
     window_tree_host_ = std::move(window_tree_host);
     root_ = window_tree_host_->window();
+    default_capture_client_ =
+        base::MakeUnique<aura::client::DefaultCaptureClient>(
+            root_->GetRootWindow());
     DCHECK(window_manager_client_);
     window_manager_client_->AddActivationParent(root_);
     ui::mojom::FrameDecorationValuesPtr frame_decoration_values =
@@ -175,6 +179,7 @@ class TestWM : public service_manager::Service,
   aura::Window* root_ = nullptr;
   aura::WindowManagerClient* window_manager_client_ = nullptr;
   std::unique_ptr<aura::WindowTreeClient> window_tree_client_;
+  std::unique_ptr<aura::client::DefaultCaptureClient> default_capture_client_;
 
   bool started_ = false;