Break a single histogram into two

Break a single histogram into two histograms

This histogram records a networking result that if positive indicates a
byte count, and if negative indicates an error code. Since histograms
can't handle negative values, this splits that up into two: one for the
byte count, and one that will use an existing enum for the network
error code.

This was based off of a suggestion in codereview.chromium.org/236023005

BUG=360909
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=264323

[Alexei Svitkine (slow), 2014-04-16 14:21:49]

LGTM

[asargent_no_longer_on_chrome, 2014-04-16 14:41:11]

The CQ bit was checked by asargent@chromium.org

[commit-bot: I haz the power, 2014-04-16 14:41:17]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/asargent@chromium.org/235893017/20001

[commit-bot: I haz the power, 2014-04-16 15:29:28]

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

[commit-bot: I haz the power, 2014-04-16 15:29:28]

Try jobs failed on following builders:
  tryserver.chromium on mac_chromium_rel

[asargent_no_longer_on_chrome, 2014-04-16 18:03:45]

The CQ bit was checked by asargent@chromium.org

[commit-bot: I haz the power, 2014-04-16 18:04:19]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/asargent@chromium.org/235893017/40001

[commit-bot: I haz the power, 2014-04-16 22:14:50]

Change committed as 264323