[unittests] Fix build/namespaces style guide violation.

[unittests] Fix build/namespaces style guide violation.

From the Google C++ style guide: "You may not use a using-directive to
make all names from a namespace available". This would be covered by
presubmit linter checks if build/namespaces were not blacklisted.

R=bmeurer@chromium.org

Committed: https://crrev.com/d8ceb9cb58932dfe52cdd4fe7331d1e0328d58a9
Cr-Commit-Position: refs/heads/master@{#31565}

[Benedikt Meurer, 2015-10-26 13:21:54]

lgtm

[Michael Starzinger, 2015-10-26 13:29:01]

The CQ bit was checked by mstarzinger@chromium.org

[commit-bot: I haz the power, 2015-10-26 13:29:11]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1410073004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1410073004/1

[commit-bot: I haz the power, 2015-10-26 13:46:37]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-26 13:47:26]

Patchset 1 (id:??) landed as
https://crrev.com/d8ceb9cb58932dfe52cdd4fe7331d1e0328d58a9
Cr-Commit-Position: refs/heads/master@{#31565}