Change GetHeap() in set methods to Heap::FromWritableHeapObject() since
the object can't be read-only if set is being called on it. Also use the
non-isolate form of set_the_hole to remove one GetIsolate().

Bug: v8:7786
Change-Id: I85cb95c8c7eb3baa9316b4a39eecf21bb13e5958
Reviewed-on: https://chromium-review.googlesource.com/1128081
Commit-Queue: Dan Elphick <delphick@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54301}