[unified-heap] Fix indexdb liveness layout tests

Replace direct GC calls that expect objects to die with asyncGC calls
that are guaranteed to run without stack.

Bug: chromium:843903
Change-Id: Ie708567fae153d0f1a20109fe170025afdc1715b
Reviewed-on: https://chromium-review.googlesource.com/1238451
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593551}
5 files changed