[zip] Add ZipReader::Next()

This method is not used yet.

It will make ZipReader easier to use, in a more idiomatic way with fewer
error checking points. It will eventually replace the methods HasMore(),
OpenCurrentEntryInZip(), AdvanceToNextEntry() and current_entry_info().

BUG=chromium:1295127
TEST=autoninja -C out/Default zlib_unittests && out/Default/zlib_unittests

Change-Id: Ibe8732ccb29f628b168c587503a90dc7d66010ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3448626


Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: François Degros <fdegros@chromium.org>
Cr-Commit-Position: refs/heads/main@{#969684}
NOKEYCHECK=True
GitOrigin-RevId: 394d2f7b127a685a82a6323c1549ebdae3b49347