sheriff: Disable flaky SystemHealthBenchmarkSmokeTest stories on Nexus5X.

The failures are caused by an internal WebSocketException, details
in bug linked.

TBR=eyaich@chromium.org

Bug: 964960
Change-Id: I1ed9697988164151519eb0b2b1fbb56884f51061
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1619829
Commit-Queue: Madeleine Barowsky <mbarowsky@chromium.org>
Reviewed-by: Madeleine Barowsky <mbarowsky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661363}
1 file changed