External references are encoded as a tuple of type and ID. This
requires both the external reference encode and the decoder to
create a mapping between the encoding and the external reference
table index.

Instead, we simply use the external reference table index as
encoding.

We now also assume that there are no duplicate entries. Existing
duplicates have been removed in this change.

R=vogelheim@chromium.org

Review URL: https://codereview.chromium.org/982773003

Cr-Commit-Position: refs/heads/master@{#27033}