The code slot of internal frames seems to be basically unused.
As always, there are exceptions:

1. In elements.cc we check whether the current code object is the apply
builtin. We can use a heap lookup through the frame's pc instead.
2. In isolate.cc we store a reference to the frame's code object to try
and pack it into the minidump. This can safely be skipped.

Remaining use-sites in frames.cc all skip INTERNAL frames by using the
JavaScriptFrameIterator.

Bug: v8:6666
Change-Id: I93c5035812838bbae5109415450915db12497b9c
Reviewed-on: https://chromium-review.googlesource.com/1075047
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53409}