This CL removes mMaxLevel from ImageHelper class. Instead, it now uses
front end's max level information when possible.

Bug: b/181800403
Change-Id: Ie0f6bd11e3ca0d4ddfc98f21261396c4d71b7140
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2796153
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Ian Elliott <ianelliott@google.com>