metrics: Add FilesystemVerifierError metric.

metrics: Add FilesystemVerifierError metric.

In https://chromium-review.googlesource.com/#/c/267554/ we added a new
error code to ChromeOS update engine. This adds FilesystemVerifierError
to histograms.xml.

BUG=None
TEST=unit tests, pretty_print.py.
R=deymo@chromium.org

Committed: https://crrev.com/4fcb3769ecc8943b0478cca396846db0936f2399
Cr-Commit-Position: refs/heads/master@{#327222}

[deymo, 2015-04-27 23:29:21]

deymo@chromium.org changed reviewers:
+ asvitkine@chromium.org, isherman@chromium.org

[deymo, 2015-04-27 23:29:21]

sounds good. CC some owners for approval.

[Ilya Sherman, 2015-04-28 00:24:58]

lgtm

[Ilya Sherman, 2015-04-28 00:25:25]

The CQ bit was checked by isherman@chromium.org

[commit-bot: I haz the power, 2015-04-28 00:26:29]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1108843005/1

[commit-bot: I haz the power, 2015-04-28 02:35:27]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-28 02:36:13]

Patchset 1 (id:??) landed as
https://crrev.com/4fcb3769ecc8943b0478cca396846db0936f2399
Cr-Commit-Position: refs/heads/master@{#327222}