Disable smoothness.top_25_smooth/youtube on Nexus 5X

NOTRY=true
TBR=vmiura@chromium.org

Bug: 812628
Change-Id: I14bd8253b5ff00f5febe4303ae3745999faa5975
Reviewed-on: https://chromium-review.googlesource.com/923579Reviewed-by: Charlie Andrews <charliea@chromium.org>
Commit-Queue: Charlie Andrews <charliea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537405}

tools/perf/expectations.config

@@ -160,6 +160,7 @@ crbug.com/528474 [ All ] smoothness.top_25_smooth/http://www.cnn.com [ Skip ]
 crbug.com/762165 [ Win ] smoothness.top_25_smooth/https://plus.google.com/110031535020051778989/posts [ Skip ]
 crbug.com/762165 [ Win ] smoothness.top_25_smooth/https://www.google.com/search?q=cats&tbm=isch [ Skip ]
 crbug.com/762165 [ Win ] smoothness.top_25_smooth/Docs_(1_open_document_tab) [ Skip ]
+crbug.com/812628 [ Nexus_5X ] smoothness.top_25_smooth/http://www.youtube.com [ Skip ]
 
 # Benchmark: smoothness.tough_ad_cases
 crbug.com/555089 [ Android_Svelte ] smoothness.tough_ad_cases/* [ Skip ]