Add a separate GetIsolate implementation for Context and JSObjects
so that we can continue using it on these context-specific types,
while eventually removing HeapObject::GetIsolate().

Bug: v8:7786
Change-Id: I86c2e12c6878e060338cbd5c3bbe27dcf978e34b
Reviewed-on: https://chromium-review.googlesource.com/1101320
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53746}