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

Suspecting Commit#
https://chromium.googlesource.com/chromium/src/+/4d8733fe8c6e16b9bd8605bb15b0003df39d11e4

@fserb -- Could you please look into the issue, kindly re-assign if this has nothing to do with your changes.
Thank You.

Fixed. on review.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/bc66507fe84e55673529822f21ee15c041430791

commit bc66507fe84e55673529822f21ee15c041430791
Author: Fernando Serboncini <fserb@chromium.org>
Date: Wed Sep 27 20:56:41 2017

Check for CSS properties before using them on FontStyleResolver

Bug:  765719 
Change-Id: Id172d596202bab6c2e8093b285feacceadc4425a
Reviewed-on: https://chromium-review.googlesource.com/672665
Commit-Queue: Fernando Serboncini <fserb@chromium.org>
Reviewed-by: Dominik Röttsches <drott@chromium.org>
Reviewed-by: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504766}
[modify] https://crrev.com/bc66507fe84e55673529822f21ee15c041430791/third_party/WebKit/Source/core/BUILD.gn
[modify] https://crrev.com/bc66507fe84e55673529822f21ee15c041430791/third_party/WebKit/Source/core/css/resolver/FontStyleResolver.cpp
[add] https://crrev.com/bc66507fe84e55673529822f21ee15c041430791/third_party/WebKit/Source/core/css/resolver/FontStyleResolverTest.cpp

ClusterFuzz has detected this issue as fixed in range 504724:504793.

Detailed report: https://clusterfuzz.com/testcase?key=6151548723003392

Fuzzer: mbarbella_js_mutation_layout
Job Type: windows_asan_content_shell
Platform Id: windows

Crash Type: Null-dereference READ
Crash Address: 0x00000000
Crash State:
  blink::StyleBuilderConverterBase::ConvertFontSize
  blink::FontStyleResolver::ComputeFont
  blink::OffscreenCanvasRenderingContext2D::setFont
  
Sanitizer: address (ASAN)

Regressed: https://clusterfuzz.com/revisions?job=windows_asan_content_shell&range=491007:491035
Fixed: https://clusterfuzz.com/revisions?job=windows_asan_content_shell&range=504724:504793

Reproducer Testcase: https://clusterfuzz.com/download?testcase_id=6151548723003392

See https://github.com/google/clusterfuzz-tools 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 6151548723003392 is verified as fixed, so closing issue as verified.

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