This expose v8::Context::DetachGlobal() as Realm.detachGlobal() in d8,
so that we can test API behaviors on Blink's detached iframes.

Change-Id: I4de1dd1a20d5862aba709e36d1d6afa9b221df6c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1554322Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60649}