The GetReadOnlyRoots will need to remain on HeapObject due to pointer
compression and the Isolate overload of HashSeed is convenient.

Change-Id: Id7fa8e57e3b8bfa6f50465c24da1fc5d0b4db0bd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2397889
Commit-Queue: Dan Elphick <delphick@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Auto-Submit: Dan Elphick <delphick@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69729}