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In turn, the compiler can then figure out some constants that are
declared extern are actually unused, so delete them,

2020 version of https://crrev.com/589575.

Change-Id: I7d98698f5124e8cd3f2f20611ea7e12b0f64e666
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2024507
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736617}