I think the reason is that convertToBlob doesn't handle very large OffscreenCanvas at this moment.

ClusterFuzz has detected this issue as fixed in range 441510:441524.

Detailed report: https://cluster-fuzz.appspot.com/testcase?key=4536150976102400

Fuzzer: inferno_twister
Job Type: mac_asan_content_shell
Platform Id: mac

Crash Type: UNKNOWN READ
Crash Address: 0x000000000020
Crash State:
  blink::WebGLRenderingContextBase::toImageData
  blink::OffscreenCanvas::convertToBlob
  blink::OffscreenCanvasV8Internal::convertToBlobMethodCallback
  
Sanitizer: address (ASAN)

Regressed: https://cluster-fuzz.appspot.com/revisions?job=mac_asan_content_shell&range=434599:434604
Fixed: https://cluster-fuzz.appspot.com/revisions?job=mac_asan_content_shell&range=441510:441524

Minimized Testcase (0.13 Kb):
Download: https://cluster-fuzz.appspot.com/download/AMIfv97LO7I21TJr2diwL3-y2jUknkouSMgULsgQl20EpfZkngq5XLfA5Qpjn0eZj9QAe_uZWn1GTmbpIwgFo5diAg9Mo0jGVoozH5GXpeODmLBn_2fCkiSesv29LQ5J2E3LoPNVYFgJV6WfRoFNqcrtw13Sn87bYQ?testcase_id=4536150976102400
<script>
  var __v_0 = new OffscreenCanvas(1265734062, 129);
  var __v_1 = __v_0.getContext('webgl');
  __v_0.convertToBlob()
</script>


See https://dev.chromium.org/Home/chromium-security/bugs/reproducing-clusterfuzz-bugs 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 4536150976102400 is verified as fixed, so closing issue.

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