Predator and CL could not provide any possible suspects.
Using Code Search for the file, "sequenced_task_runner_handle.cc" assigning to the concern owner.

Suspecting Commit#
https://chromium.googlesource.com/chromium/src/+/f22c9807f7c040510a7de490be613b6f803cc62b

@gab -- Could you please look into the issue, kindly re-assign if this is not related to your changes.
Thank You.

Same issue as 760412 but for rsa_import_key_pkcs8_fuzzer.cc instead of rsa_import_key_spki_fuzzer.cc

ClusterFuzz has detected this issue as fixed in range 501346:501422.

Detailed report: https://clusterfuzz.com/testcase?key=5925543584464896

Fuzzer: libFuzzer_webcrypto_rsa_import_key_pkcs8_fuzzer
Job Type: libfuzzer_chrome_msan
Platform Id: linux

Crash Type: CHECK failure
Crash Address: 
Crash State:
  pool. Error: This caller requires a sequenced context (i.e. the current task nee
  SignalHandler
  base::SequencedTaskRunnerHandle::Get
  
Sanitizer: memory (MSAN)

Regressed: https://clusterfuzz.com/revisions?job=libfuzzer_chrome_msan&range=498138:498181
Fixed: https://clusterfuzz.com/revisions?job=libfuzzer_chrome_msan&range=501346:501422

Reproducer Testcase: https://clusterfuzz.com/download?testcase_id=5925543584464896

See https://chromium.googlesource.com/chromium/src/+/master/testing/libfuzzer/reproducing.md for more information.

If you suspect that the result above is incorrect, try re-doing that job on the test case report page.