Serializer sees uninitialized memory inside some BigInt. PTAL

Fix: https://chromium-review.googlesource.com/#/c/v8/v8/+/946676

The following revision refers to this bug:
  https://chromium.googlesource.com/v8/v8.git/+/824358f07bacb4a4d5d58defbae8490cfa0fa85c

commit 824358f07bacb4a4d5d58defbae8490cfa0fa85c
Author: Jakob Kummerow <jkummerow@chromium.org>
Date: Fri Mar 02 20:06:57 2018

[bigint] Make MSan happy: zero-initialize unused bits

There are some unused bits in a BigInt's bit field. We never read their
their values explicitly, but whenever the entire object is moved around
(for serialization, or GC), this uninitialized memory is accessed. This
patch fixes that by initializing the entire field after allocation of a
BigInt, not just the bits we actually use.

Bug:  chromium:818109 
Change-Id: I5a4d24c3240242157b902c696fa9bb779799280d
Reviewed-on: https://chromium-review.googlesource.com/946676
Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51708}
[modify] https://crrev.com/824358f07bacb4a4d5d58defbae8490cfa0fa85c/src/objects/bigint.cc

Should be fixed by #4.

For the record, this did not have security implications.

ClusterFuzz has detected this issue as fixed in range 51707:51708.

Detailed report: https://clusterfuzz.com/testcase?key=5714112219447296

Fuzzer: mbarbella_js_mutation
Job Type: linux_msan_d8
Platform Id: linux

Crash Type: Use-of-uninitialized-value
Crash Address: 
Crash State:
  v8::internal::Serializer<v8::internal::DefaultSerializerAllocator>::ObjectSerial
  v8::internal::Serializer<v8::internal::DefaultSerializerAllocator>::ObjectSerial
  v8::internal::Serializer<v8::internal::DefaultSerializerAllocator>::ObjectSerial
  
Sanitizer: memory (MSAN)

Recommended Security Severity: Medium

Regressed: https://clusterfuzz.com/revisions?job=linux_msan_d8&range=51672:51673
Fixed: https://clusterfuzz.com/revisions?job=linux_msan_d8&range=51707:51708

Reproducer Testcase: https://clusterfuzz.com/download?testcase_id=5714112219447296

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 5714112219447296 is verified as fixed, so closing issue as verified.

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