Disable same-origin-subframe.sub.html

Disable the test virtual/threaded-composited-iframes/external/wpt/
is-input-pending/tentative/same-origin-subframe.sub.html on Mac due to
flaky.

Bug: 1124978
TBR: npm@chromium.org
Change-Id: I8c690da0f9ab0d456534d353716e708cfe12f54a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2399300Reviewed-by: Nicolás Peña Moreno <npm@chromium.org>
Reviewed-by: Owen Min <zmin@chromium.org>
Commit-Queue: Owen Min <zmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805049}

third_party/blink/web_tests/TestExpectations

@@ -6622,3 +6622,6 @@ crbug.com/1122582 external/wpt/html/cross-origin-opener-policy/coop-csp-sandbox-
 
 # Sheriff 2020-09-02
 crbug.com/1115091 [ Linux ] external/wpt/payment-request/secure-payment-confirmation.https.html [ Pass Timeout ]
+
+# Sheriff 2020-09-08
+crbug.com/1124978 [ Mac ] virtual/threaded-composited-iframes/external/wpt/is-input-pending/tentative/same-origin-subframe.sub.html [ Pass Failure ]