[blink_perf.css] Disable failing test CustomPropertiesVarAlias.html

NOTRY=true
TBR=nednguyen@google.com
CC=andruud@chromium.org,futhark@chromium.org

Bug: 891878
Change-Id: I52e2b98faee15e90b2d6dbb71f1d572fa64c5ab7
Reviewed-on: https://chromium-review.googlesource.com/c/1259617Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Commit-Queue: Caleb Rouleau <crouleau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596373}

tools/perf/expectations.config

@@ -26,6 +26,9 @@ crbug.com/853738 [ Android_Webview ] blink_perf.canvas/draw-video-to-hw-accelera
 crbug.com/853738 [ Android_Webview ] blink_perf.canvas/upload-video-to-sub-texture.html [ Skip ]
 crbug.com/853738 [ Android_Webview ] blink_perf.canvas/upload-video-to-texture.html [ Skip ]
 
+# Benchmark: blink_perf.css
+crbug.com/891878 [ Nexus5X_Webview ] blink_perf.css/CustomPropertiesVarAlias.html [ Skip ]
+
 # Benchmark: blink_perf.layout
 crbug.com/551950 [ Android_Svelte ] blink_perf.layout/* [ Skip ]
 crbug.com/832686 [ Nexus_5 ] blink_perf.layout/subtree-detaching.html [ Skip ]