This manages input_index directly in TranslatedFrame::iterator.
I think the overhead is low enough, expecially since all uses
of the iterator, except one, compute input_index anyway.

Bug: v8:7679
Change-Id: I7e5fc08ff23a49415265afd617248c55f4d95e19
Reviewed-on: https://chromium-review.googlesource.com/1021711
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52739}