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Only build seed corpora where it is necessary, in order to reduce
build sizes.

Bug: 952412
Change-Id: I6e529226d4ba51fd547c1869525a9f094fdcd998
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1566454Reviewed-by: Max Moroz <mmoroz@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Jonathan Metzman <metzman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650506}