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

Suspecting Commit#
https://chromium.googlesource.com/chromium/src/+/15d1b34174529968cac05a8902c96368687a8fd4

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

I don't think this DCHECK has anything to do with my change mentioned in comment 1. kqyang@ could you take a look? According to git blame that DCHECK was added by your CL.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/19c84d8d09af498626eb71e2f47f02837c50ee23

commit 19c84d8d09af498626eb71e2f47f02837c50ee23
Author: KongQun Yang <kqyang@chromium.org>
Date: Tue Aug 29 23:19:12 2017

Remove the DCHECK on group_description_index

It is possible that the input file has sample to group box but does
not have a corresponding sample group description box, in which case
|entries| is empty.

Bug:  760037 
Change-Id: I89f4e0c439c2ab211505f21e2bf7d4eff07a1385
Reviewed-on: https://chromium-review.googlesource.com/642125
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Commit-Queue: Kongqun Yang <kqyang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#498277}
[modify] https://crrev.com/19c84d8d09af498626eb71e2f47f02837c50ee23/media/formats/mp4/track_run_iterator.cc

ClusterFuzz has detected this issue as fixed in range 498217:498277.

Detailed report: https://clusterfuzz.com/testcase?key=5219410951536640

Fuzzer: libFuzzer_mediasource_MP4_AVC1_pipeline_integration_fuzzer
Job Type: libfuzzer_chrome_asan_debug
Platform Id: linux

Crash Type: CHECK failure
Crash Address: 
Crash State:
  group_description_index <= entries->size() in track_run_iterator.cc
  base::debug::DebugBreak
  media::mp4::GetSampleEncryptionInfoEntry
  
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=498217:498277

Reproducer Testcase: https://clusterfuzz.com/download?testcase_id=5219410951536640

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 5219410951536640 is verified as fixed, so closing issue as verified.

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