Revert "sheriff: Disable flaky SystemHealthBenchmarkSmokeTest stories on Nexus5X."

This reverts commit 28b151ee.

Reason for revert: Reverting to confirm the nature of the original errors.

Original change's description:
> 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}

TBR=mbarowsky@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 964960
Change-Id: Ibb7f61efd2fcbb4ed05f3724a4b4984325929ec1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1627034Reviewed-by: Juan Antonio Navarro Pérez <perezju@chromium.org>
Reviewed-by: Madeleine Barowsky <mbarowsky@chromium.org>
Commit-Queue: Juan Antonio Navarro Pérez <perezju@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662588}

tools/perf/expectations.config

@@ -317,10 +317,6 @@ crbug.com/923527 [ Android_Webview ] system_health.memory_mobile/load:media:soun
 crbug.com/947267 [ Nexus_5X ] system_health.memory_mobile/background:media:imgur [ Skip ]
 crbug.com/954949 [ Nexus5X_Webview ] system_health.memory_mobile/browse:news:washingtonpost [ Skip ]
 crbug.com/961417 [ Android_Go ] system_health.memory_mobile/browse:social:tumblr_infinite_scroll:2018 [ Skip ]
-crbug.com/964960 [ Nexus_5X ] system_health.memory_mobile/load:media:soundcloud:2018 [ Skip ]
-crbug.com/964960 [ Nexus_5X ] system_health.memory_mobile/load:social:twitter [ Skip ]
-crbug.com/964960 [ Nexus_5X ] system_health.memory_mobile/load:media:facebook_photos [ Skip ]
-crbug.com/964960 [ Nexus_5X ] system_health.memory_mobile/load:search:yahoo:2018 [ Skip ]
 
 crbug.com/964804 [ Nexus_5X ] system_health.memory_mobile/browse:media:flickr_infinite_scroll [ Skip ]
 crbug.com/964804 [ Nexus_5X ] system_health.memory_mobile/browse:search:amp:2018 [ Skip ]