WTF's unit test helpers should be usable from other unit tests in Blink.

This patch adds a new gyp target called "wtf_unittest_helpers", which should
contain unittest-only utilities for WTF types. The target can be used not only
from wtf_unittests but from other unit tests in Blink.

Originally, WTFTestHelpers was a header-only utility, thus it could be used
from any modules in Blink, including production codes. This patch tightens up
the situation a bit; if a function in wtf_unittest_helpers is used from
a non-unittest module, that will result a link error.

Unit-test only patch; no behavior change.

BUG=none
R=morrita@chromium.org

Review URL: https://codereview.chromium.org/90893004

git-svn-id: svn://svn.chromium.org/blink/trunk@162871 bbb929c8-8fbe-4397-9dbb-9b2b20218538