Generating custom previews can invoke user specified JavaScript (via the
`window.devtoolsFormatters` custom formatters feature). These custom
formatters were previously invoked in the main page context, even for
objects coming from other `<iframe>`s. Instead of using the main
renderer context, we should instead generate the custom preview in the
creation context of the object.

Bug: chromium:997925
Change-Id: Ia07915cff6680153b6727e68117ed565e60bc1c2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1776093Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63476}