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We still want to preserve the following histograms:
* PrefetchedSignedExchangeCache.HeadersSizeTotal
* Net.NetworkErrorLogging.SignedExchangeRequestOutcome

Bug: 1165258, 1165256
Change-Id: If7d302b8298e89a1d2a164b8f81a459725e1431b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2626617
Commit-Queue: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Auto-Submit: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843023}