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

Suspecting Commit#
https://chromium.googlesource.com/chromium/src/+/7c451e21433ae47ea740b116d6095bb824781d86

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

bcwhite@: Might you know what's going on here?

The faulting line is:

  UMA_HISTOGRAM_ENUMERATION("Navigation.MainFrameScheme", scheme, SCHEME_MAX);

... and hasn't changed in a long time. That macro expands to:

  define UMA_HISTOGRAM_ENUMERATION(name, sample, enum_max) \
  INTERNAL_HISTOGRAM_ENUMERATION_WITH_FLAG(               \
      name, sample, enum_max, base::HistogramBase::kUmaTargetedHistogramFlag)

... which itself invokes other macros.

However, within the regression range, we find https://chromium.googlesource.com/chromium/src/+/82027ff5dafbb3fbafe44fede998a33667ea2bd1%5E%21/#F0 which changed some of the histogram serialization to return a nullptr, in scenarios where "a compromised renderer manages to send an invalid serialized histogram" (per https://bugs.chromium.org/p/chromium/issues/detail?id=753222#c7) 

What's less clear to me is how the browser is "protected" from that invalid histogram, since it's going to operate on that nullptr and crash anyway?

Serialization is for handling histograms passed via IPC from a subprocess like the Renderer.  That's not the case here.

Any chance this metric could have been previously created with different parameters?

Re #3: Interesting. 

When you say "with different parameters", can you help me understand what you mean? The CL in #1 added new values to the 'scheme' enumeration and increased the value of 'SCHEME_MAX' accordingly, but no other changes were made.

ClusterFuzz testcase 5614535728955392 is flaky and no longer crashes, so closing issue.

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