SocketDescriptor should not be used as a boolean

In UnixDomainServerSocketTest.AcceptWithForbiddenUser, a
SocketDescriptor value was being tested as a boolean, which
shouldn't happen. Instead, compare to kInvalidSocket.

Also, build trivial external_application_unittests on Windows.

BUG=None
TEST=mojob.py test
R=jam@chromium.org, davemoore, jam

Review URL: https://codereview.chromium.org/736443002