When we add waitAsync, void* backing_store_ is no longer sufficient,
since we cannot guarantee that the BackingStore stays alive while
we're (asynchronously) waiting.

Bug: v8:10239
Change-Id: I0e4f6049eb0c01faf216a8c6865a41c6e42be3ac
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2208854Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67922}