Mark fast/selectors/placeholder-shown-style-update.html as crashing.

Only crashes on ASAN. There's no good way to express that in the test
expectations so it's [ Pass Crash ]

TBR=smcgruer@chromium.org

Bug: 1121429
Change-Id: Ia702dbbcf3175e17098c07f8f3b5271b4e2379be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2371831Reviewed-by: Fergal Daly <fergal@chromium.org>
Commit-Queue: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801279}

third_party/blink/web_tests/TestExpectations

@@ -6709,3 +6709,6 @@ crbug.com/1117613 media/video-error-networkState.html [ Failure Timeout ]
 crbug.com/1120330 virtual/threaded/external/wpt/feature-policy/experimental-features/vertical-scroll-disabled-scrollbar-tentative.html [ Pass Failure ]
 
 crbug.com/1119676 external/wpt/html/cross-origin-embedder-policy/reporting-to-endpoint.https.html [ Failure ]
+
+# Sheriff 2020-08-25
+crbug.com/1121429 fast/selectors/placeholder-shown-style-update.html [ Pass Crash ]