Pass Isolate directly into several LayoutDescriptor methods so they
don't call GetIsolate on unsafe objects.

Also marks DebugInfo as non-read-only (so our GetIsolate removal tools
stop trying to change BreakIterator::isolate() to call itself).

Bug: v8:7786
Change-Id: I626a83d603ab74f648c72eb50d027b3866cedceb
Reviewed-on: https://chromium-review.googlesource.com/1138326
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54490}