That way the code calling setVisible knows it can safely create a
framebuffer for this window without having its content be undefined.

BUG=angleproject:830

Change-Id: Ia694f50ac20e1d510d0c0f899226ed241d9a5a3a
Reviewed-on: https://chromium-review.googlesource.com/271156Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>