This removes mBaseLevel from ImageHelper class. With the
mFirstAllocatedLevel tracking exactly which GL level has been allocated,
this cached mBaseLevel is no longer needed.

Bug: b/181800403
Change-Id: I99d66c93b0c8f1bd20a5811b51f512a27927201e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2795275
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Ian Elliott <ianelliott@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>