Turn `debug::EntriesPreview` into a public API.
This is a straightforward approach to addressing
https://github.com/nodejs/node/issues/20409
(not relying on functionality behind `--allow-natives-syntax`)
in Node.js.

Refs: https://github.com/nodejs/node/issues/20409
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I7021e5846012a55a82c488408ded6591f6b139e7
Reviewed-on: https://chromium-review.googlesource.com/1057467Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53226}