Refactor a SocketStream unittest.

Some review comments at http://crrev.com/23496078/ remained to be addressed.
This CL addresses them.

BUG=290957
R=tyoshino

Review URL: https://codereview.chromium.org/26225002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227443 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227443