Prior to a mutation of the bitset, mCurrentIterator would have been
from mParent but post-mutation subsequent iterators are obtained
from mParentCopy. Assert that the end() markers for both containers
are the same.

Bug: angleproject:3877
Change-Id: Ia29abd2acc0014522ef5856a88d8ec227cf9f19d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2693813Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>