Minimize headless code that refers to aura.

headless/lib/browser/headless_window_tree_host.cc

Index: headless/lib/browser/headless_window_tree_host.cc
diff --git a/headless/lib/browser/headless_window_tree_host.cc b/headless/lib/browser/headless_window_tree_host.cc
index 96cec1a96e69b017d71a78010989d9fc01685a75..5cae74e7d48a2cf1e5d37efe3b82211db4444035 100644
--- a/headless/lib/browser/headless_window_tree_host.cc
+++ b/headless/lib/browser/headless_window_tree_host.cc
@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 #include "headless/lib/browser/headless_window_tree_host.h"
+#include "ui/aura/window.h"
 
 #include "ui/gfx/icc_profile.h"
 
@@ -15,10 +16,15 @@ HeadlessWindowTreeHost::HeadlessWindowTreeHost(const gfx::Rect& bounds)
 }
 
 HeadlessWindowTreeHost::~HeadlessWindowTreeHost() {
+  window_parenting_client_.reset();
   DestroyCompositor();
   DestroyDispatcher();
 }
 
+void HeadlessWindowTreeHost::SetParentWindow(gfx::NativeWindow window) {
+  window_parenting_client_.reset(new HeadlessWindowParentingClient(window));
+}
+
 bool HeadlessWindowTreeHost::CanDispatchEvent(const ui::PlatformEvent& event) {
   return false;
 }