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

Committed: https://crrev.com/dbecf20d65460a2856563b46f6f0267805cae64f
Cr-Commit-Position: refs/heads/master@{#26994}

[Yang, 2015-03-04 14:41:51]

On 2015/03/04 14:39:45, Yang wrote:

The snapshot blob shrinks very slightly after this.

[vogelheim, 2015-03-04 15:04:17]

lgtm

[Yang, 2015-03-04 15:06:43]

The CQ bit was checked by yangguo@chromium.org

[commit-bot: I haz the power, 2015-03-04 15:07:16]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/974273002/1

[commit-bot: I haz the power, 2015-03-04 15:36:06]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-04 15:36:14]

Patchset 1 (id:??) landed as
https://crrev.com/dbecf20d65460a2856563b46f6f0267805cae64f
Cr-Commit-Position: refs/heads/master@{#26994}