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This is an unused manifest key which hasn't been used for
years. It seems this was used for signature verification:
https://codereview.chromium.org/118328/patch/1205/130 but at
some point it's use was removed.

TODO: Also remove this key from
https://developer.chrome.com/docs/extensions/mv2/manifest/ and
https://developer.chrome.com/docs/extensions/mv3/manifest/.

BUG=None

Change-Id: I4ed42510e169c2fca569362820ebb210a1082845
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2622572Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843235}