[Sheriff] Disable external/wpt/infrastructure/server/wpt-server-wpt-flags.sub.html

This test is failing on Mac and Win bots with an image
diff, following the import at
https://chromium-review.googlesource.com/c/chromium/src/+/2584498.

Sample: https://ci.chromium.org/p/chromium/builders/ci/Mac10.14%20Tests/11475

Looks like the failure is due to "/opt" vs the expected "/b" in paths
that appear in the error image.

Bug: 626703
Change-Id: I4fe0f1a309bd397c9a1509e1d01010aec9512f3b
Notry: true
Tbr: robertma@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2587654Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836315}

third_party/blink/web_tests/TestExpectations

@@ -5918,3 +5918,5 @@ crbug.com/1157861 http/tests/devtools/console/paintworklet-console-selector.js [
 # Wpt importer Sheriff 2020-12-11
 crbug.com/626703 [ Linux ] wpt_internal/storage/estimate-usage-details-filesystem.https.tentative.any.html [ Pass Failure ]
 crbug.com/626703 [ Mac ] external/wpt/mediacapture-image/MediaStreamTrack-getConstraints.https.html [ Pass Failure ]
+crbug.com/626703 [ Mac ] external/wpt/infrastructure/server/wpt-server-wpt-flags.sub.html [ Pass Failure ]
+crbug.com/626703 [ Win ] external/wpt/infrastructure/server/wpt-server-wpt-flags.sub.html [ Pass Failure ]