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

Suspecting Commit#
https://chromium.googlesource.com/chromium/src/+/2a88dac7dd3caf5155d4e82f08c86d12484dbf18

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

ClusterFuzz has detected this issue as fixed in range 496881:496921.

Detailed report: https://clusterfuzz.com/testcase?key=5433156844126208

Fuzzer: marty_html_twiddler
Job Type: windows_asan_chrome_no_sandbox
Platform Id: windows

Crash Type: Null-dereference READ
Crash Address: 0x00000000
Crash State:
  blink::Scrollbar::ConvertFromRootFrameToParentView
  blink::ScrollbarTheme::HitTestWithRootFramePoint
  blink::Scrollbar::MouseMoved
  
Sanitizer: address (ASAN)

Regressed: https://clusterfuzz.com/revisions?job=windows_asan_chrome_no_sandbox&range=495902:496126
Fixed: https://clusterfuzz.com/revisions?job=windows_asan_chrome_no_sandbox&range=496881:496921

Reproducer Testcase: https://clusterfuzz.com/download?testcase_id=5433156844126208

Additional requirements: Requires Gestures

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.