This is owned by the ImageHelper. We'll use it in tracking used images
in RenderPasses.

Bug: angleproject:4911
Change-Id: Ic71cef2b16eefb86c73c9c5e1e38ee7cd6a60adb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2332895
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>