Make NodeIterator more STL-friendly.
Make NodeIterator more STL-friendly.

This allows it to be used in various algorithms.

BUG=
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_site_isolation

Review URL: https://codereview.chromium.org/1728253003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#381807}

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