The serializer does not support serialization of HashTables in general because
after deserialization it might be necessary to rehash the table.
However the UnseededNumberDictionary does not require rehashing and this CL allows
them to be serialized.

This CL also changes the shape of UnseededNumberDictionary: the details field is
no longer part of the entry since no one needs it.

BUG=chromium:576312, chromium:623516

Review-Url: https://codereview.chromium.org/2102073002
Cr-Commit-Position: refs/heads/master@{#37336}