[zone-containers] Clean up ZoneChunkList iterators

Merge all of them to a single implementation, getting rid of all the
duplication.

R=marja@chromium.org

Change-Id: I5201e81ec64f3d7789df5e72bf58c85231cb348c
Reviewed-on: https://chromium-review.googlesource.com/868133
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50685}