This will let us store references to Surface in FBO attachments,
even after the surface is destroyed.

BUG=angleproject:963

Change-Id: I7e1cb161d1e08f78b1c4d730a32ad09ac7e61e30
Reviewed-on: https://chromium-review.googlesource.com/263482Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>