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

Suspecting Commit#
https://chromium.googlesource.com/chromium/src/+/9cddc0b9cd174242d365e9d64615687384b73c89

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

I have a local repro using the fuzzer case.
Concerningly, the line before this DCHECK(false) is:
// This should not happen if WebMListParser is working properly.

Investigating...

=> xhwang@ :

It looks like at least kWebMIdContentSigAlgo is *not* handled properly when the webm parser finds it.  In fact, I don't see any place in our code where that EBML Tag ID is even used other than being defined and defined as a valid UINT member of a list kWebMIdContentEncryption pieces.

In debug builds -> DCHECK
In release builds -> parse error

I'm not at all clear on what the real correct behavior for Chrome should be though. It seems from the line right before the DCHECK, though, that this DCHECK's failure is unexpected.

Please take a look. Thanks!

I'll take a look

Given this is a DCHECK and in release mode we handle the error correctly, I am going to move this issue to P2.

Automatically applying components based on information from OWNERS files. If this seems incorrect, please apply the Test-Predator-Wrong-Components label.

For more information, please see https://chromium.googlesource.com/chromium/src/+/master/testing/libfuzzer/reference.md.

The link referenced in the description is no longer valid.

ClusterFuzz testcase 6291972330618880 appears to be flaky, updating reproducibility label.

Please ignore the last comment about testcase being unreproducible. The testcase is still reproducible. This happened due to a code refactoring on ClusterFuzz side, and the underlying root cause is now fixed. Resetting the label back to Reproducible. Sorry about the inconvenience caused from these incorrect notifications.

Please ignore the last comment about testcase being unreproducible. The testcase is still reproducible. This happened due to a code refactoring on ClusterFuzz side, and the underlying root cause is now fixed. Resetting the label back to Reproducible. Sorry about the inconvenience caused from these incorrect notifications.

ClusterFuzz has detected this issue as fixed in range 621021:621022.

Detailed report: https://clusterfuzz.com/testcase?key=6291972330618880

Fuzzer: libFuzzer_mediasource_WEBM_VP9_pipeline_integration_fuzzer
Fuzz target binary: mediasource_WEBM_VP9_pipeline_integration_fuzzer
Job Type: libfuzzer_chrome_asan_debug
Platform Id: linux

Crash Type: CHECK failure
Crash Address: 
Crash State:
  false in webm_content_encodings_client.cc
  media::WebMContentEncodingsClient::OnUInt
  media::ParseUInt
  
Sanitizer: address (ASAN)

Regressed: https://clusterfuzz.com/revisions?job=libfuzzer_chrome_asan_debug&range=497039:497107
Fixed: https://clusterfuzz.com/revisions?job=libfuzzer_chrome_asan_debug&range=621021:621022

Reproducer Testcase: https://clusterfuzz.com/download?testcase_id=6291972330618880

See https://github.com/google/clusterfuzz-tools for instructions to reproduce this bug locally.

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

ClusterFuzz testcase 6291972330618880 is verified as fixed, so closing issue as verified.

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