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

ash/mus/window_manager_unittest.cc

Index: ash/mus/window_manager_unittest.cc
diff --git a/ash/mus/window_manager_unittest.cc b/ash/mus/window_manager_unittest.cc
index 4c2c0cd0e88246778ebfd775b94812bdbd1a1108..168bb4b702fb215d3163cd0f267df51e08f28a84 100644
--- a/ash/mus/window_manager_unittest.cc
+++ b/ash/mus/window_manager_unittest.cc
@@ -49,6 +49,9 @@
   void OnLostConnection(aura::WindowTreeClient* client) override {}
   void OnPointerEventObserved(const ui::PointerEvent& event,
                               aura::Window* target) override {}
+  aura::client::CaptureClient* GetCaptureClient() override {
+    return wm_state_.capture_controller();
+  }
   aura::PropertyConverter* GetPropertyConverter() override {
     return &property_converter_;
   }