Refactor ImageIndex to have separate type and target members. BUG=angleproject:2169 Change-Id: Ib3fb699058f76d0eb810a9691ea1d64311dadbb1 Reviewed-on: https://chromium-review.googlesource.com/929650 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>