Move parent of D3D surface window off the main GPU thread

gpu/ipc/service/child_window_surface_win.h

Index: gpu/ipc/service/child_window_surface_win.h
diff --git a/gpu/ipc/service/child_window_surface_win.h b/gpu/ipc/service/child_window_surface_win.h
index 2915d2b10b725d8d1d48e256dcba18c11334cb40..ba8d35005920953c7fb8dc39f01b80e949d91e65 100644
--- a/gpu/ipc/service/child_window_surface_win.h
+++ b/gpu/ipc/service/child_window_surface_win.h
@@ -36,7 +36,11 @@ class ChildWindowSurfaceWin : public gl::NativeViewGLSurfaceEGL {
   // This member contains all the data that can be accessed from the main or
   // window owner threads.
   std::unique_ptr<SharedData> shared_data_;
+  // The eventual parent of the window living in the browser process.
   HWND parent_window_;
+  // The window is initially created with this parent window. We need to keep it
+  // around so that we can destroy it at the end.
+  HWND initial_parent_window_;
   GpuChannelManager* manager_;
   bool alpha_;
   bool first_swap_;