We might run TRACE_GC with ThreadKind::kMain not only on each isolate's
main thread but also on the shared isolate's thread during a shared GC.
The DCHECK is too restrictive for the latter case. This is safe because
the shared GC will stop all main threads before starting its work.

Bug: v8:11708
Change-Id: I1f40140d6502b1ec797dfa783fb693ed213efb3b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3380522Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78588}