In snapshots with several contexts, some contexts may not reference
function or object templates, and therefore would not require external
references for deserialization. However, function and object templates
are deserialized with the isolate as part of the partial snapshot cache,
so we would need these external references even if we only use contexts
that don't need them.

With this patch, we use a fallback in case no external references are
provided. This way, we only run into issues when we actually call native
callbacks.

R=jgruber@chromium.org, peria@chromium.org

Change-Id: I6af8a77f26c92bd73fdab6112474c62da270597f
Reviewed-on: https://chromium-review.googlesource.com/784831Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49615}