This is a reland of 3b93e695

Original change's description:
> Add InstanceType for EphemeronHashTable
>
> This will allow the GC to differentiate between regular HashTables and EphemeronHashTables.
>
> Bug: chromium:844008
> Change-Id: I2f9009ac25eb117de03786b110dd362b829c5e9e
> Reviewed-on: https://chromium-review.googlesource.com/1089066
> Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Commit-Queue: Dominik Inführ <dinfuehr@google.com>
> Cr-Commit-Position: refs/heads/master@{#53577}

TBR=tebbi@chromium.org

Bug: chromium:844008
Change-Id: Ia5da3d15e28ace94063f9db4fdd4997d88696501
Reviewed-on: https://chromium-review.googlesource.com/1091090
Commit-Queue: Dominik Inführ <dinfuehr@google.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53594}