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/+/1762488Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688629}

tools/perf/expectations.config

@@ -327,6 +327,7 @@ 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 ]