Happily this also removes a use of GetIsolate().

Also delete Descriptor::Init which is no longer needed and some friend
declarations.

Bug: v8:7786
Change-Id: Ica803ef6c8963b38aef72121ff4fb737c9780920
Reviewed-on: https://chromium-review.googlesource.com/1133982Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54394}