Fix serialization statistics for external strings.

Fix serialization statistics for external strings.

External strings are serialized as internal strings. The current way
of collecting stats is slightly wrong. We ought to use the map and
size passed to SerializePrologue.

R=jkummerow@chromium.org

Committed: https://crrev.com/e96d25536ea2a84459ff5cec531727e8f53ca5d9
Cr-Commit-Position: refs/heads/master@{#27931}

[Yang, 2015-04-20 06:48:05]

The CQ bit was checked by yangguo@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-04-20 06:48:17]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1092253003/1

[commit-bot: I haz the power, 2015-04-20 07:32:34]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-04-20 07:32:35]

Dry run: This issue passed the CQ dry run.

[Jakob Kummerow, 2015-04-20 08:52:15]

If you say so...

LGTM.

[Yang, 2015-04-20 08:54:28]

The CQ bit was checked by yangguo@chromium.org

[commit-bot: I haz the power, 2015-04-20 08:55:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1092253003/1

[commit-bot: I haz the power, 2015-04-20 08:55:21]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-20 08:55:34]

Patchset 1 (id:??) landed as
https://crrev.com/e96d25536ea2a84459ff5cec531727e8f53ca5d9
Cr-Commit-Position: refs/heads/master@{#27931}