Revert: Remove WindowTreeClientDelegate::GetCaptureClient() and WindowTreeClient::GetCaptureClient()

ui/aura/mus/window_tree_client.h

Index: ui/aura/mus/window_tree_client.h
diff --git a/ui/aura/mus/window_tree_client.h b/ui/aura/mus/window_tree_client.h
index 09dc89bab29ccbacaefc2ba0ac59b57d560f5635..b1514b578c58b69d36e5c159944c007f7ce51c3f 100644
--- a/ui/aura/mus/window_tree_client.h
+++ b/ui/aura/mus/window_tree_client.h
@@ -67,6 +67,10 @@
 class WindowTreeClientObserver;
 class WindowTreeClientTestObserver;
 class WindowTreeHostMus;
+
+namespace client {
+class CaptureClient;
+}
 
 using EventResultCallback = base::Callback<void(ui::mojom::EventResult)>;
 
@@ -110,6 +114,8 @@
 
   bool connected() const { return tree_ != nullptr; }
   ClientSpecificId client_id() const { return client_id_; }
+
+  client::CaptureClient* GetCaptureClient();
 
   void SetCanFocus(Window* window, bool can_focus);
   void SetCanAcceptDrops(Id window_id, bool can_accept_drops);