Add unknown dimension for data use histograms

Add unknown dimension for data use histograms

Following CL changed data use histograms to be logged as foreground,
background and unknown. This CL adds the dimensions to histograms.xml.
https://crrev.com/b07005bcaac418bfed5e0e3a93edb025c8e43dfa

BUG=648808
Committed: https://crrev.com/4a5e8956c85b5ac72858d08cd472babc89e9c9b9
Cr-Commit-Position: refs/heads/master@{#428314}

[Raj, 2016-10-27 21:08:43]

rajendrant@chromium.org changed reviewers:
+ rkaplow@chromium.org

[rkaplow, 2016-10-28 01:13:22]

lgtm

[Raj, 2016-10-28 06:58:08]

The CQ bit was checked by rajendrant@chromium.org

[commit-bot: I haz the power, 2016-10-28 06:58:19]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-10-28 07:57:21]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-10-28 08:01:02]

Description was changed from

==========
Add unknown dimension for data use histograms

Following CL changed data use histograms to be logged as foreground,
background and unknown. This CL adds the dimensions to histograms.xml.
https://crrev.com/b07005bcaac418bfed5e0e3a93edb025c8e43dfa

BUG=648808
==========

to

==========
Add unknown dimension for data use histograms

Following CL changed data use histograms to be logged as foreground,
background and unknown. This CL adds the dimensions to histograms.xml.
https://crrev.com/b07005bcaac418bfed5e0e3a93edb025c8e43dfa

BUG=648808

Committed: https://crrev.com/4a5e8956c85b5ac72858d08cd472babc89e9c9b9
Cr-Commit-Position: refs/heads/master@{#428314}
==========

[commit-bot: I haz the power, 2016-10-28 08:01:03]

Patchset 1 (id:??) landed as
https://crrev.com/4a5e8956c85b5ac72858d08cd472babc89e9c9b9
Cr-Commit-Position: refs/heads/master@{#428314}