The serializer performs two passes over the code. The first pass copies out the
code content verbatim, the second pass visits references recorded in the reloc
info.

So far the first pass is implicit and happens as part of the second pass, when
we encounter a non-HeapObject reference when iterating the code object.  That
however does not work for internal references. So we hit an assertion if the
first non-HeapObject reference we see is an internal reference.

This change explicitly triggers the first pass.

R=petermarshall@chromium.org

Bug: v8:6817
Change-Id: I1ee9949e10b7d9409986da83be22ac6287785f9f
Reviewed-on: https://chromium-review.googlesource.com/663867Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48010}