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
Review-Url: https://codereview.chromium.org/2445193008
Cr-Commit-Position: refs/heads/master@{#428314}
(cherry picked from commit 4a5e8956c85b5ac72858d08cd472babc89e9c9b9)

Committed: https://chromium.googlesource.com/chromium/src/+/17c2ae517b4d62a3a409d43fe4f2d1c868004172

[Raj, 2016-10-31 03:25:53]

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

Review-Url: https://codereview.chromium.org/2445193008
Cr-Commit-Position: refs/heads/master@{#428314}
(cherry picked from commit 4a5e8956c85b5ac72858d08cd472babc89e9c9b9)
==========

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

Review-Url: https://codereview.chromium.org/2445193008
Cr-Commit-Position: refs/heads/master@{#428314}
(cherry picked from commit 4a5e8956c85b5ac72858d08cd472babc89e9c9b9)

Committed:
https://chromium.googlesource.com/chromium/src/+/17c2ae517b4d62a3a409d43fe4f2...
==========

[Raj, 2016-10-31 03:25:54]

Committed patchset #1 (id:1) manually as
17c2ae517b4d62a3a409d43fe4f2d1c868004172.