All HeapObjects already have roots access so this was redundant and
made ComputeAndSetHash difficult to use.
Eventually we need to get rid of the Isolate version of HashSeed,
but this will touch a lot of files, so leaving it for now.

Bug: v8:8562
Change-Id: I27d8fe10df72494d0a2146f408a2158cf02ce226
Reviewed-on: https://chromium-review.googlesource.com/c/1481630
Commit-Queue: Maciej Goszczycki <goszczycki@google.com>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Reviewed-by: Mythri Alle <mythria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59792}