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

Suspecting Commit#
https://chromium.googlesource.com/chromium/src/+/c81b9ab9c23121da0539e13ab3b6fca9b08b37c8

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

@tkent, can you please have a look?

A local bisect points to https://chromium-review.googlesource.com/588728, which changes some code in the immediate vicinity of the crash location.

It's super easy to reproduce. On e.g. x64 debug, run content_shell with the minimized repro from Clusterfuzz. It doesn't reproduce 100% reliably, but for me it seems to always repro after two or three tries.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0d862af0161839aac40d476966db504ebe940ab4

commit 0d862af0161839aac40d476966db504ebe940ab4
Author: Kent Tamura <tkent@chromium.org>
Date: Wed Aug 02 05:39:59 2017

Fix a crash by adding <base> to a detached srcdoc document.

ParentDocument() can be nullptr.

Bug:  750836 
Change-Id: Idff5feeececa8ad32f3248c565c878902dd24ffc
Reviewed-on: https://chromium-review.googlesource.com/597370
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491277}
[modify] https://crrev.com/0d862af0161839aac40d476966db504ebe940ab4/third_party/WebKit/LayoutTests/html/README.md
[add] https://crrev.com/0d862af0161839aac40d476966db504ebe940ab4/third_party/WebKit/LayoutTests/html/infrastructure/fallback_base_url-expected.txt
[add] https://crrev.com/0d862af0161839aac40d476966db504ebe940ab4/third_party/WebKit/LayoutTests/html/infrastructure/fallback_base_url.html
[modify] https://crrev.com/0d862af0161839aac40d476966db504ebe940ab4/third_party/WebKit/Source/core/dom/Document.cpp

ClusterFuzz has detected this issue as fixed in range 491244:491285.

Detailed report: https://clusterfuzz.com/testcase?key=5536586535272448

Fuzzer: ifratric-browserfuzzer-v3
Job Type: windows_asan_chrome_no_sandbox
Platform Id: windows

Crash Type: Null-dereference READ
Crash Address: 0x00000208
Crash State:
  blink::KURL::IsNull
  blink::Document::FallbackBaseURL
  blink::Document::ProcessBaseElement
  
Sanitizer: address (ASAN)

Regressed: https://clusterfuzz.com/revisions?job=windows_asan_chrome_no_sandbox&range=490252:490299
Fixed: https://clusterfuzz.com/revisions?job=windows_asan_chrome_no_sandbox&range=491244:491285

Reproducer Testcase: https://clusterfuzz.com/download?testcase_id=5536586535272448


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 5536586535272448 is verified as fixed, so closing issue as verified.

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