- Iterator advancing is kept mainly unchanged.
- The iterator stores the size of the object which is to be used by the
  caller in follow ups. This way we might be able to avoid further out
  of line loads.
- The iteartor follows the regular std conventions allowing range based
  loops.

Bug: chromium:651354
Change-Id: I8928224a62d3a48a48145a2d00279a28608bc634
Reviewed-on: https://chromium-review.googlesource.com/543335
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46085}