Now that vk::ImageHelper is stored as a pointer, it can be shared between an
offscreen surface and texture.

BUG=angleproject:3073

Change-Id: I91e520259106eef497950b8b2e622fbf910f7444
Reviewed-on: https://chromium-review.googlesource.com/c/1412234
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>