Disable failing blink_perf.events story on 5X webview

NOTRY=true
TBR=nednguyen@chromium.org

Bug: 851539
Change-Id: I2128758cdc151cf496ee9485d9f639641bb48c69
Reviewed-on: https://chromium-review.googlesource.com/1095636
Commit-Queue: Charlie Andrews <charliea@chromium.org>
Reviewed-by: Charlie Andrews <charliea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#566040}

tools/perf/expectations.config

@@ -27,6 +27,7 @@ crbug.com/738453 [ Nexus_6 ] blink_perf.canvas/putImageData.html [ Skip ]
 
 # Benchmark: blink_perf.events
 crbug.com/799184 [ Android_One ] blink_perf.events/EventsDispatchingInShadowTrees.html [ Skip ]
+crbug.com/851539 [ Nexus5X_Webview ] blink_perf.events/EventsDispatchingInShadowTrees.html [ Skip ]
 
 # Benchmark: blink_perf.layout
 crbug.com/551950 [ Android_Svelte ] blink_perf.layout/* [ Skip ]