Revert "sheriff: mark tumblr_infinite_scroll flaky"

This reverts commit 688cb8a3.

Reason for revert: Underlying issue fixed with https://chromium-review.googlesource.com/c/v8/v8/+/1782169.

Original change's description:
> sheriff: mark tumblr_infinite_scroll flaky
> 
> It was already marked flaky in desktop_common but needs to be marked as
> such in desktop_memory as well.
> 
> TBR=monicaciocea@chromium.org,eyaich@chromium.org
> 
> Bug: 859500, 995813
> Change-Id: I2d93395a23d987ec11a2e3cd6d231d952ff50b23
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1762488
> Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
> Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#688629}

TBR=ellyjones@chromium.org

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

Bug: 859500, 995813
Change-Id: Id26acefd175b13f42b8b9f3f0e4b2468b79a3913
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1789310Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694820}

tools/perf/expectations.config

@@ -331,7 +331,6 @@ crbug.com/934885 [ linux ] system_health.memory_desktop/load_accessibility:media
 crbug.com/869118 [ linux ] system_health.memory_desktop/long_running:tools:gmail-background [ Skip ]
 crbug.com/836447 [ chromeos ] system_health.memory_desktop/multitab:misc:typical24 [ Skip ]
 crbug.com/899887 [ linux ] system_health.memory_desktop/browse:social:facebook_infinite_scroll:2018 [ Skip ]
-crbug.com/859500 [ linux ] system_health.memory_desktop/browse:social:tumblr_infinite_scroll:2018 [ Skip ]
 crbug.com/924330 [ linux ] system_health.memory_desktop/browse:media:pinterest:2018 [ Skip ]
 crbug.com/874803 [ win10 ] system_health.memory_desktop/multitab:misc:typical24 [ Skip ]
 crbug.com/944978 [ win7 ] system_health.memory_desktop/multitab:misc:typical24 [ Skip ]