Mark editing/pasteboard/pasteboard_with_unfocused_selection.html flaky

TBR=odejesush@chromium.org
NOTRY=true

Bug: 1047293
Change-Id: Ie5fc08657a7675f7030d8ad5779d2c5f1c792d6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2031553Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736948}

third_party/blink/web_tests/TestExpectations

@@ -6509,3 +6509,4 @@ crbug.com/1047126 [ Win ] external/wpt/IndexedDB/keypath-exceptions.htm [ Failur
 crbug.com/1047164 [ Mac ] fast/scroll-snap/snaps-after-keyboard-scrolling.html [ Pass Failure ]
 crbug.com/1047208 virtual/omt-service-worker-startup/http/tests/serviceworker/update-served-from-cache.html [ Pass Failure ]
 crbug.com/1047208 http/tests/serviceworker/update-served-from-cache.html [ Pass Failure ]
+crbug.com/1047293 [ Mac ] editing/pasteboard/pasteboard_with_unfocused_selection.html [ Pass Failure ]