Minimize headless code that refers to aura.

headless/lib/browser/headless_window_tree_host.h

Index: headless/lib/browser/headless_window_tree_host.h
diff --git a/headless/lib/browser/headless_window_tree_host.h b/headless/lib/browser/headless_window_tree_host.h
index c01545c9c35fc4e4c395c0281ce679c96e4c113a..0aadfc87d18c37737ea8d0e198d1611373714ce1 100644
--- a/headless/lib/browser/headless_window_tree_host.h
+++ b/headless/lib/browser/headless_window_tree_host.h
@@ -5,7 +5,10 @@
 #ifndef HEADLESS_LIB_BROWSER_HEADLESS_WINDOW_TREE_HOST_H_
 #define HEADLESS_LIB_BROWSER_HEADLESS_WINDOW_TREE_HOST_H_
 
+#include <memory>
+
 #include "base/macros.h"
+#include "headless/lib/browser/headless_window_parenting_client.h"
 #include "ui/aura/window_tree_host.h"
 #include "ui/events/platform/platform_event_dispatcher.h"
 #include "ui/gfx/geometry/rect.h"
@@ -18,6 +21,8 @@ class HeadlessWindowTreeHost : public aura::WindowTreeHost,
   explicit HeadlessWindowTreeHost(const gfx::Rect& bounds);
   ~HeadlessWindowTreeHost() override;
 
+  void SetParentWindow(gfx::NativeWindow window);
+
   // ui::PlatformEventDispatcher:
   bool CanDispatchEvent(const ui::PlatformEvent& event) override;
   uint32_t DispatchEvent(const ui::PlatformEvent& event) override;
@@ -39,6 +44,7 @@ class HeadlessWindowTreeHost : public aura::WindowTreeHost,
 
  private:
   gfx::Rect bounds_;
+  std::unique_ptr<aura::client::WindowParentingClient> window_parenting_client_;
 
   DISALLOW_COPY_AND_ASSIGN(HeadlessWindowTreeHost);
 };