[Sheriff] Update editing/selection/selection-background.html expectations on Mac.

flake portal:
https://analysis.chromium.org/p/chromium/flake-portal/flakes/occurrences?key=ag9zfmZpbmRpdC1mb3ItbWVyTwsSBUZsYWtlIkRjaHJvbWl1bUBibGlua193ZWJfdGVzdHNAZWRpdGluZy9zZWxlY3Rpb24vc2VsZWN0aW9uLWJhY2tncm91bmQuaHRtbAw

TBR=wangxianzhu@chromium.org

Bug: 1104910
Change-Id: I017a9e76b54725bafa403c55927efc5e00e41ee5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2303113
Commit-Queue: Sahel Sharify <sahel@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Sahel Sharify <sahel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789170}

third_party/blink/web_tests/TestExpectations

@@ -6861,7 +6861,7 @@ crbug.com/1104910 [ Mac ] external/wpt/webaudio/the-audio-api/the-oscillatornode
 crbug.com/1105957 fast/dom/cssTarget-crash.html [ Pass Timeout ]
 crbug.com/1104910 fast/peerconnection/RTCPeerConnection-reload-interesting-usage.html [ Pass Failure ]
 crbug.com/1104910 [ Win ] virtual/cache-storage-eager-reading/external/wpt/service-workers/service-worker/registration-updateviacache.https.html [ Pass Failure ]
-crbug.com/1104910 [ Mac10.15 ] editing/selection/selection-background.html [ Pass Failure ]
+crbug.com/1104910 [ Mac ] editing/selection/selection-background.html [ Pass Failure ]
 crbug.com/1104910 [ Linux ] external/wpt/referrer-policy/gen/worker-module.http-rp/unsafe-url/worker-classic.http.html [ Pass Failure ]
 crbug.com/1104910 [ Linux ] external/wpt/referrer-policy/gen/worker-module.http-rp/unset/worker-module.http.html [ Pass Failure ]