Serializer: fix false negative in hashmap lookups.

Serializer: fix false negative in hashmap lookups.

If we use HashMap::Lookup with insert=true, the returned entry may have
NULL as value. This could either mean that the value is 0, or that the
entry has just been inserted. This ambiguity can cause false negatives
in PartialCacheIndexMap::LookupOrInsert.

Also fix a TODO.

R=vogelheim@chromium.org

Review URL: https://codereview.chromium.org/974273002

Cr-Commit-Position: refs/heads/master@{#26994}