Introduce CSVPasswordIterator

As described in the design doc [1], a new class, CSVPasswordIterator,
is introduced as an abstraction for consuming CSV data line by line,
representing each line as a CSVPassword.

[1] https://docs.google.com/document/d/1wsZBl93S_WGaXZqrqq5SP08LVZ0zDKf6e9nlptyl9AY/edit?usp=sharing

Bug: 934326
Change-Id: I3477a600abad1c41f27a2ebca6c9c150da560818
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1700049
Commit-Queue: Vaclav Brozek <vabr@chromium.org>
Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684267}