[vm] Fix one race reported in DEBUG+TSAN mode
[vm] Fix one race reported in DEBUG+TSAN mode

The SafepointHandler::IsOwnedByTheThread() is used in a few
ASSERT()/RELEASE_ASSERT() and accesses the [owner_] field
unsynchronized.

Issue https://github.com/dart-lang/sdk/issues/47788

TEST=Fixes one race reported when building TSAN in DEBUG mode.

Change-Id: I64371265d60c9bb68c299d1c1e8abc2175b33bc8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/221331
Reviewed-by: Slava Egorov <72c81f38df725d8f1832acb9b5bb99563fc2f1b9@google.com>
Commit-Queue: Martin Kustermann <a35dd5b79fac84b2bb3b026a16b8becc55af5961@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk