The serializer already has code that special cases for some external
strings. We can handle all external strings in one place instead of
splitting the logic between the serializer and the object visitor.

The main benefit is that we remove two virtual functions from the
ObjectVisitor and thus simplify it for all other users.

BUG=chromium:709075

Review-Url: https://codereview.chromium.org/2799943002
Cr-Commit-Position: refs/heads/master@{#44485}