To avoid repeating code for the few places where we can call GetIsolate
and GetHeap safely.

Bug: v8:7786
Change-Id: I6c6de81488bfa79dca50cfd2b6356f432401b68e
Reviewed-on: https://chromium-review.googlesource.com/1104684
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53828}