validation_pool: reduce the throttling backoff lookback threshold

If we start failing a lot of runs in a row, the exponential backoff of
CLs picked up gets so agressive as to make the CQ basically useless.

Reduce the history lookback so that this is less aggressive.

BUG=None
TEST=None

Change-Id: I4de48194630e4893969bff3ebc18583e31dd3e0e
Reviewed-on: https://chromium-review.googlesource.com/446624
Commit-Queue: Aviv Keshet <akeshet@chromium.org>
Tested-by: Aviv Keshet <akeshet@chromium.org>
Trybot-Ready: Aviv Keshet <akeshet@chromium.org>
Reviewed-by: Ningning Xia <nxia@chromium.org>
1 file changed