We don't need to store the native context explicitly anymore, the
broker already has it.

Bug: v8:7790
Change-Id: I1096953e3c56bed9d3a8d7d37b108888ef4ac7ec
Reviewed-on: https://chromium-review.googlesource.com/c/1270594
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56586}