Resets the isolate's context to nullptr in debug builds during bytecode finalization
to ensure that we don't rely on the native context during context independent
unoptimized compilation.

BUG=chromium:898076, v8:8041

Change-Id: Ifaa5006a7a3d31d7fbd535ebb63f8889c75526c4
Reviewed-on: https://chromium-review.googlesource.com/c/1297961
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56979}