Disable toBlob_duration.html on nexus 5x webview.

This test is flaky on the nexus 5x webview. This is due to a timeout
Exception. This was recently added in http://crrev/c/2367753.

No-Try: True
TBR: johnchen@chromium.org
CC: behdadb@chromium.org,jonross@chromium.org,sadrul@chromium.org,yiyix@chromium.org
Bug: 1124237
Change-Id: I9086c4f1a7b443fe35924cad15ea287bc6259c47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2390063Reviewed-by: Stephen Nusko <nuskos@chromium.org>
Commit-Queue: Stephen Nusko <nuskos@chromium.org>
Auto-Submit: Stephen Nusko <nuskos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803825}

tools/perf/expectations.config

@@ -246,6 +246,7 @@ crbug.com/1017244 [ mobile ] rendering.mobile/youtube_2018 [ Skip ]
 crbug.com/1036357 [ android-nexus-5 ] rendering.mobile/idle_power_blank [ Skip ]
 crbug.com/1036357 [ android-nexus-5 ] rendering.mobile/balls_svg_animations [ Skip ]
 crbug.com/1116469 [ android-webview ] rendering.mobile/webgl_to_texture [ Skip ]
+crbug.com/1124237 [ android-nexus-5x android-webview ] rendering.mobile/toBlob_duration.html [ Skip ]
 
 # Benchmark: rasterize_and_record_micro.top_25
 crbug.com/764543 rasterize_and_record_micro.top_25/file://static_top_25/wikipedia.html [ Skip ]