Predator and CL could not provide any possible suspects.
Using Code Search for the file, "JBig2_Context.cpp" assigning to the concern owner using GIT Blame.

Suspecting Commit#
https://pdfium.googlesource.com/pdfium.git/+/18531230381465592e1a037e6dfc39d4a6fb5def

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

The following revision refers to this bug:
  https://pdfium.googlesource.com/pdfium/+/fd70d79ca67ac87dd95ab23d548b1fcb879ad259

commit fd70d79ca67ac87dd95ab23d548b1fcb879ad259
Author: Nicolas Pena <npm@chromium.org>
Date: Wed Aug 30 20:07:56 2017

Fix undefined shift in CJBig2_Context::decodeSymbolIDHuffmanTable

Bug:  chromium:755532 
Change-Id: Ib04426fab52d0ca1d2544a21fd2ce4faaa57123f
Reviewed-on: https://pdfium-review.googlesource.com/12430
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Nicolás Peña <npm@chromium.org>

[modify] https://crrev.com/fd70d79ca67ac87dd95ab23d548b1fcb879ad259/core/fxcodec/jbig2/JBig2_Context.cpp

ClusterFuzz has detected this issue as fixed in range 498575:498629.

Detailed report: https://clusterfuzz.com/testcase?key=5533834031661056

Fuzzer: libFuzzer_pdfium_fuzzer
Job Type: libfuzzer_chrome_ubsan
Platform Id: linux

Crash Type: Undefined-shift
Crash Address: 
Crash State:
  CJBig2_Context::decodeSymbolIDHuffmanTable
  CJBig2_Context::parseTextRegion
  CJBig2_Context::parseSegmentData
  
Sanitizer: undefined (UBSAN)

Regressed: https://clusterfuzz.com/revisions?job=libfuzzer_chrome_ubsan&range=395640:395746
Fixed: https://clusterfuzz.com/revisions?job=libfuzzer_chrome_ubsan&range=498575:498629

Reproducer Testcase: https://clusterfuzz.com/download?testcase_id=5533834031661056

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 5533834031661056 is verified as fixed, so closing issue as verified.

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