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/+/2507830
Reviewed-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}