Disable flaky browse:social:pinterest_infinite_scroll

Test is flaky on Nexus 5X

TBR=perezju@chromium.org

Bug: 859500
Change-Id: Ide9926b503a4f2b7faeaba3791694cff69b0cf81
Reviewed-on: https://chromium-review.googlesource.com/1123260
Commit-Queue: Sarah Chan <spqchan@chromium.org>
Reviewed-by: Sarah Chan <spqchan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572021}

tools/perf/expectations.config

@@ -298,6 +298,7 @@ crbug.com/834905 [ Nexus_5 ] system_health.memory_mobile/browse:social:pinterest
 crbug.com/842731 [ Android_One ] system_health.memory_mobile/background:news:nytimes [ Skip ]
 crbug.com/843547 [ Android_Go ] system_health.memory_mobile/background:news:nytimes [ Skip ]
 crbug.com/852888 [ Nexus_5X ] system_health.memory_mobile/background:news:nytimes [ Skip ]
+crbug.com/859500 [ Nexus_5X ] system_health.memory_mobile/browse:social:tumblr_infinite_scroll [ Skip ]
 
 # Benchmark: tab_switching.typical_25
 crbug.com/747026 [ Mac ] tab_switching.typical_25/multitab:misc:typical24 [ Skip ]