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

ui/aura/mus/window_tree_client_delegate.h

Index: ui/aura/mus/window_tree_client_delegate.h
diff --git a/ui/aura/mus/window_tree_client_delegate.h b/ui/aura/mus/window_tree_client_delegate.h
index e76d2efbac0b596f543f1e0765080ee391e24b38..609a05785854c6fd8accfccc5d02dfdd67f333df 100644
--- a/ui/aura/mus/window_tree_client_delegate.h
+++ b/ui/aura/mus/window_tree_client_delegate.h
@@ -18,6 +18,10 @@
 class Window;
 class WindowTreeClient;
 class WindowTreeHostMus;
+
+namespace client {
+class CaptureClient;
+}
 
 // Interface implemented by an application using mus.
 class AURA_EXPORT WindowTreeClientDelegate {
@@ -58,6 +62,10 @@
   virtual void OnPointerEventObserved(const ui::PointerEvent& event,
                                       Window* target) = 0;
 
+  // Mus expects a single CaptureClient is used for all WindowTreeHosts. This
+  // returns it. GetCaptureClient() is called from the constructor.
+  virtual client::CaptureClient* GetCaptureClient() = 0;
+
   virtual PropertyConverter* GetPropertyConverter() = 0;
 
  protected: