PrototypeUsers::Add now iterates the WeakArrayList to find empty slots
before growing the array. Not reusing empty slots caused a memory leak.

It might also be desirable to shrink the WeakArrayList in the future.
Right now it is only compacted when invoking CreateBlob.

Also removed unused PrototypeUsers::IsEmptySlot declaration.

Bug: v8:10031
Change-Id: I570ec78fca37e8f0c794f1f40846a4daab47c225
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1967317Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65456}