This requires serialized data to track the number of API-provided
external references separately.
And it flushes out a case of serialized data corruption (stored "length"
field too large) that we didn't handle without crashing.

BUG=v8:6055

Review-Url: https://codereview.chromium.org/2736923002
Cr-Commit-Position: refs/heads/master@{#43649}