Bisect points to 6cb995b936d63ce651273ff420d3dbf80ff71f13, and the testcase looks pretty much the same as the added test cases for that CL.

Jakob, can you take a look?

The following revision refers to this bug:
  https://chromium.googlesource.com/v8/v8.git/+/ceba405f28a6b20814107930365788f2749db574

commit ceba405f28a6b20814107930365788f2749db574
Author: jkummerow <jkummerow@chromium.org>
Date: Fri May 05 22:23:04 2017

[runtime] MigrateFastToFast: fix check for unboxed inobject doubles

After the recent fast-property deletion changes, there can be a
non-empty out-of-object backing store (that previously held properties)
even though the next double property will be stored in-object.

BUG= chromium:718779 

Review-Url: https://codereview.chromium.org/2861093004
Cr-Commit-Position: refs/heads/master@{#45146}

[modify] https://crrev.com/ceba405f28a6b20814107930365788f2749db574/src/objects.cc
[add] https://crrev.com/ceba405f28a6b20814107930365788f2749db574/test/mjsunit/regress/regress-crbug-718779.js

ClusterFuzz has detected this issue as fixed in range 45145:45146.

Detailed report: https://clusterfuzz.com/testcase?key=4901273202851840

Fuzzer: mbarbella_js_mutation
Job Type: linux_asan_d8_dbg
Platform Id: linux

Crash Type: CHECK failure
Crash Address: 
Crash State:
  !new_map->IsUnboxedDoubleField(index) in objects.cc
  
Sanitizer: address (ASAN)

Regressed: V8: 45068:45069
Fixed: V8: 45145:45146

Reproducer Testcase: https://clusterfuzz.com/download?testcase_id=4901273202851840


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 4901273202851840 is verified as fixed, so closing issue.

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