Serializer: correctly deal with internal references.

Serializer: correctly deal with internal references.

Internal references are absolute addresses into the instruction
stream. Turn them into relative addresses when serializing and
back when deserializing to keep them valid.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/976623002

Cr-Commit-Position: refs/heads/master@{#27020}