Since GC can now happen during deserialization, object fields may
contain the Smi sentinel value instead of pointers. This adds the
required guards to methods of NativeContextInferrer

Bug: chromium:1136801
Change-Id: I7338f31bf6ee34b8dee8431b8250d2cc2978e0c2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2461241
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70425}