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This bumps it from 2 to 4 across every bot. This helps builders like the
amd64-generic CQ bot which runs shards with 15-17min runtimes
(https://ci.chromium.org/p/chromium/builders/try/chromeos-amd64-generic-rel/737112)
and the eve CI bot which runs shards with ~30min runtimes
(https://ci.chromium.org/p/chrome/builders/ci/chromeos-eve-chrome/12083).

Bug: None
Change-Id: I3ef9d7eaabf3d1d8085d55b4f25adc70a6fc3f03
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2622897Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842318}