Disable v8.browsing_mobile/browse:media:imgur:2019 on pixel2 webview

This is currently 12.5% flaky and is already disabled on nexus5x
webview.

This is due to story occasionally generating to much data for the trace
buffer.

Bug: 1143740
Change-Id: I74bc5fdaad999bce76d8a469b8470175c307b26d
No-Try: True
TBR: johnchen@chromium.org
CC: mythria@chromium.org,tmrts@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2507830Reviewed-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@{#822263}

tools/perf/expectations.config

@@ -444,6 +444,7 @@ crbug.com/1064284 [ android-pixel-2 ] v8.browsing_mobile/browse:social:pinterest
 crbug.com/1088419 [ android-nexus-5x android-webview ] v8.browsing_mobile/browse:media:imgur:2019 [ Skip ]
 crbug.com/1112337 [ android ] v8.browsing_mobile/browse:news:cricbuzz:2019 [ Skip ]
 crbug.com/1139057 [ mobile ] v8.browsing_mobile/browse:social:facebook:2019 [ Skip ]
+crbug.com/1143740 [ android-pixel-2 android-webview ] v8.browsing_mobile/browse:media:imgur:2019 [ Skip ]
 
 # Benchmark: v8.browsing_mobile-future
 crbug.com/1036141 [ android-webview ] v8.browsing_mobile-future/browse:shopping:lazada:2019 [ Skip ]