Uses Heap::FromWritableHeapObject where the object must be mutable and
otherwise pass Isolate in. String and subclasses should now be free of
GetIsolate/GetHeap.

Bug: v8:7786
Change-Id: Ic9d55250ea4afebbebda87be9415ccdaa4fc8779
Reviewed-on: https://chromium-review.googlesource.com/1131736Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54395}