Texture serials were not used in any place that pointers could not be used
and Texture::INCOMPLETE_TEXTURE_ID was only referenced by RendererD3D.

This was the last place that serials are used in the gl layer.

BUG=angleproject:681

Change-Id: Iad24321fba57db8ade3c9eb8f04cc20e98ded1b0
Reviewed-on: https://chromium-review.googlesource.com/263659Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>