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.

Testcase 6620083282771968 is a top crash on ClusterFuzz for linux platform. Please prioritize fixing this crash.

Marking this crash as a stable release blocker. If this is incorrect, remove the ReleaseBlock label.

The issue is that LLVMFuzzerTestOneInput() -> webcrypto::ImportRsaKeyFromDerFuzzData() eventually lands in mojo which requires SequencedTaskRunnerHandle::IsSet().

This can be fixed I think by adding a mere base::MessageLoop in LLVMFuzzerTestOneInput() and potentially calling RunLoop().Run() after the crypto call.

 Issue 760410  has been merged into this issue.

https://chromium-review.googlesource.com/c/chromium/src/+/663675

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9457c5ef45a382c72ae3d28374c2ce844979dcc6

commit 9457c5ef45a382c72ae3d28374c2ce844979dcc6
Author: Eric Roman <eroman@chromium.org>
Date: Tue Sep 12 21:53:16 2017

Fix a CHECK failure when running components/webcrypto fuzzer tests.

Bug:  760412 
Change-Id: I636562f761b0574369b40b64c0e9f180a1eed1e5
Reviewed-on: https://chromium-review.googlesource.com/663675
Reviewed-by: Ryan Sleevi <rsleevi@chromium.org>
Commit-Queue: Eric Roman <eroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501415}
[modify] https://crrev.com/9457c5ef45a382c72ae3d28374c2ce844979dcc6/components/webcrypto/fuzzer_support.cc

[Auto-generated comment by a script] We noticed that this issue is targeted for M-62; it appears the fix may have landed after branch point, meaning a merge might be required. Please confirm if a merge is required here - if so add Merge-Request-62 label, otherwise remove Merge-TBD label. Thanks.

This crash was specific to the fuzz testing setup, and not something that adversely affects production code.

ClusterFuzz has detected this issue as fixed in range 501393:501473.

Detailed report: https://clusterfuzz.com/testcase?key=6620083282771968

Fuzzer: libFuzzer_webcrypto_rsa_import_key_spki_fuzzer
Job Type: libfuzzer_chrome_asan
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
  base::SequencedTaskRunnerHandle::Get
  mojo::internal::GetTaskRunnerToUseFromUserProvidedTaskRunner
  
Sanitizer: address (ASAN)

Regressed: https://clusterfuzz.com/revisions?job=libfuzzer_chrome_asan&range=498121:498180
Fixed: https://clusterfuzz.com/revisions?job=libfuzzer_chrome_asan&range=501393:501473

Reproducer Testcase: https://clusterfuzz.com/download?testcase_id=6620083282771968

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.

ClusterFuzz testcase 5925543584464896 is verified as fixed, so closing issue as verified.

If this is incorrect, please add ClusterFuzz-Wrong label and re-open the issue.