Allow ReadOnlyRoots initialization from an OffThreadHeap, by creating
a FromHeap getter on OffThreadIsolate analogous to the one on Isolate.

Bug: chromium:1075999
Change-Id: Ie00e1547160e24d35bd7b0dd36d1b7eead87341e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2184289Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67600}