NATIVE_CONTEXT_IMPORTED_FIELDS are no longer different from other
NATIVE_CONTEXT_FIELDS, so this CL joins them together.

While doing so, removed the OBJECT_VALUE_OF slot, which is not
referenced anywhere.

Change-Id: I79c46e6e6cc92c90e90e68165cf7b0ae31e8151d
Reviewed-on: https://chromium-review.googlesource.com/c/1387631Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58449}