Currently attached references are only used for global proxy, source
string and code stubs. Mid-term future we want to use attached
references for arbitrary objects (in fixed order) provided from outside.

This change renames BackReference to SerializerReference to include both
back references and attached references.

R=mtrofin@chromium.org, vogelheim@chromium.org

Review-Url: https://codereview.chromium.org/1992723002
Cr-Commit-Position: refs/heads/master@{#36318}