Predator and CL did not provide any possible suspects.
Using Code Search for the file, "CSSTokenizer.cpp" assigning to the concern owner.
Suspecting Commit#
https://chromium.googlesource.com/chromium/src/+/a081fcfbc7471a6681142d49491b82f7b8402851

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

Test case looks to be trying to make 4000 stylesheets of "zzzz.." (repeated 2^16) and the allocator doesn't like it. Don't think there's anything to do here.