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Avoid a crash caused by mutation of the list being iterated over from inside
the foreach loop. This does a full copy of the the iterated list since the
code inside can't mutate the array via the loop variable anyway. Although
theoretically slower, this doesn't seem to have a measurable performance
regression in practice (we generally iterate over few large lists).

Adds documentation and tests for iteration while mutating the underlying list
variable.

Bug: 818525
Change-Id: I221fa230685b8998f5874154cad8d5c655b8006c
Reviewed-on: https://chromium-review.googlesource.com/959228
Commit-Queue: Brett Wilson <brettw@chromium.org>
Reviewed-by: Roland McGrath <mcgrathr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544209}