[BidiCaretAffinity] Unify "offset" and "index" for NGCaretNagivator

As discussed with yosin@, this patch unifies these two concepts in most
of NGCaretNavigator to avoid confusion.

Bug: 894651
Change-Id: I3321b97e1641aaed57b65959e7344e9e5d0aca80
Reviewed-on: https://chromium-review.googlesource.com/c/1406298
Commit-Queue: Emil A Eklund <eae@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Reviewed-by: Yoichi Osato <yoichio@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622199}
6 files changed