[ScrollTimeline] Mark test element-based-offset.html as flaky

This test has been flaky under virtual/threaded test suite.
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=blink_web_tests%20(with%20patch)&tests=external%2Fwpt%2Fscroll-animations%2Felement-based-offset.html

Bug: 1080609
Change-Id: I2fce270915b921028411e144c0fef7326ad14e99
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2212361Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771019}

third_party/blink/web_tests/TestExpectations

@@ -5886,3 +5886,5 @@ crbug.com/1084509 [ Win ] fast/forms/select/select-disabled-appearance.html [ Pa
 crbug.com/1011811 http/tests/devtools/network/network-xhr-data-received-async-response-type-blob.js [ Pass Failure ]
 crbug.com/1011811 http/tests/devtools/network/download.js [ Pass Failure ]
 crbug.com/1011811 http/tests/devtools/sxg/sxg-disable-cache.js [ Pass Failure ]
+
+crbug.com/1080609 virtual/threaded/external/wpt/scroll-animations/element-based-offset.html [ Pass Failure ]