Find it and CL did not provide any possible suspects.
Using Code Search for the file, "FontFallbackIterator.cpp" assigning to the concern owner.

Suspecting Commit#
https://chromium.googlesource.com/chromium/src/+/dd927e05ce64314bec330c5ad7b744625c5ab181

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

Seems unrelated. Nothing else leaps to mind, sorry.

ClusterFuzz has detected this issue as fixed in range 445271:445273.

Detailed report: https://cluster-fuzz.appspot.com/testcase?key=5652032079003648

Fuzzer: ochang_search_index_mutator
Job Type: linux_ubsan_chrome
Platform Id: linux

Crash Type: CHECK failure
Crash Address: 
Crash State:
  lastResort in FontFallbackIterator.cpp
  blink::FontFallbackIterator::next
  blink::FontFallbackIterator::next
  
Sanitizer: undefined (UBSAN)

Regressed: https://cluster-fuzz.appspot.com/revisions?job=linux_ubsan_chrome&range=443258:443393
Fixed: https://cluster-fuzz.appspot.com/revisions?job=linux_ubsan_chrome&range=445271:445273

Unminimized Testcase: https://cluster-fuzz.appspot.com/download/AMIfv96PBkjFwI2KgEbr1_d9PfS3UApGzXG1zI7WRTwfz6ZLDK4B8fZ5mWGEEJVItu1AnSjL0GmJ7Z1X_xlDBvYZ4D1NePVolEDz09EwnU4c2Sb7g44vNrENHH6p7_Bn9PQAFtf0arQNZIMwTBy_VlG6u75n_9l7xdEMcw6in3o0nqjuydwVcLS8vKpTNygCL__ZLEEvkb7qwtcg94wLB-vEeDxz2kYc_24b3UtDfFdFQF9AeZQZq7Xlb8nuVBFXLuPFI3CATUTdYEyhA07vW7L7fWPtqsY_csOUeGbf_hnk5_fd33lltMHA4xsGNnchX9fh0YpBKPyW7_ZJGbdr6fHC-goCa2t5I2HtdgTYIPqtE73lkz416HEAH_J2Wi_q0MYFKfyHjnVE5LkSIUJpZATHs8xvR6n50A?testcase_id=5652032079003648


Additional requirements: Requires Gestures

See https://dev.chromium.org/Home/chromium-security/bugs/reproducing-clusterfuzz-bugs 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 5652032079003648 is verified as fixed, so closing issue.

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