[inspector] Skip special __proto__ property if already present.

The inspector has some magic to add a special __proto__ property to
object value mirrors as long as the object itself has a [[Prototype]].
However it doesn't check whether the object already has a regular
property named __proto__ and thus confuses the front-end by sending two
properties with the same name.

Fixed: chromium:1193250
Change-Id: I75a1cd78ba94aeda4afedcc0f1e69b8dadb6673f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2810784


Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Auto-Submit: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73836}