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This is a preparation for follow up CLs migrating to JS modules which
will use those modules in externs as JS modules instead of using as
externs. This is due to limitations on Closure compiler working with
JS modules and externs.

BUG=chromium:1133186, b/172300267

Change-Id: Ic983ba08b050f161c6b6e2e9b287f19584409982
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2517350
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Jeremie Boulic <jboulic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823917}