Fix -Wmicrosoft-unqualified-friend warning.

Fix -Wmicrosoft-unqualified-friend warning.

The warning complains that StringThreadTest is declared in the global namespace,
but the friend line says `friend class StringThreadTest;` instead of
`friend class ::StringThreadTest`.  MSVC doesn't require this ::, but gcc does
-- without it, the friend line declares a new class in the namespace containing
UnicodeString.

Since StringThreadTest was in test/threadtest which isn't built as part of
Chromium's build and which was removed in upstream ICU in ICU 37392, just
delete the friend line, it has no effect.

BUG=505296
R=jshin@chromium.org

Committed: https://chromium.googlesource.com/chromium/deps/icu/+/6b3ce817f8e828c3b7a577d2395f0882eb56ef18

[Nico, 2015-08-25 16:17:25]

thakis@chromium.org changed reviewers:
+ jshin@chromium.org

[Nico, 2015-08-25 16:17:25]

Can you upstream this change too, please?

[jungshik at Google, 2015-08-25 17:18:11]

LGTM. 

I'll upstream it. Thanks

[Nico, 2015-08-25 17:30:25]

Committed patchset #3 (id:40001) manually as
6b3ce817f8e828c3b7a577d2395f0882eb56ef18 (presubmit successful).

[jungshik at Google, 2015-08-25 21:08:32]

Upstreamed : http://bugs.icu-project.org/trac/ticket/11866