If the isolate is available via another object (including JSReceivers
and Contexts), grab it through that rather than HeapObject::GetIsolate.
As a drive-by, fix up places where this looked like it would work but
didn't because of null or uninitialized objects.

Bug: v8:7786
Change-Id: I3454840073d1455b3738e0ac34ccaa3c9a220896
Reviewed-on: https://chromium-review.googlesource.com/1104170Reviewed-by: Dan Elphick <delphick@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53876}