[Sheriff] Expect timeout for external/wpt/event-timing/bufferbeforeonload.html on Linux

TBR=npm@chromium.org,yoavweiss@chromium.org

Bug: 987236
Change-Id: I1890d52f825c8a87713f902efb2bd2ba76bea91d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715354Reviewed-by: Maxim Kolosovskiy <kolos@chromium.org>
Commit-Queue: Maxim Kolosovskiy <kolos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680415}

third_party/blink/web_tests/TestExpectations

@@ -3018,6 +3018,7 @@ crbug.com/763830 virtual/blink-cors/http/tests/security/cors-rfc1918/ [ Skip ]
 
 crbug.com/831729 external/wpt/event-timing/crossiframe.html [ Timeout ]
 crbug.com/831729 external/wpt/event-timing/observer-manual.html [ Skip ]
+crbug.com/987236  [ Linux ] external/wpt/event-timing/bufferbeforeonload.html [ Timeout ]
 
 # Working on getting the CSP tests going:
 crbug.com/694525 external/wpt/content-security-policy/connect-src/worker-from-guid.sub.html [ Skip ]