Refactored out UrlIsAlive to live outside the ApacheHttpd class and imported http_utils.py into http_server.py

Now they share common code.

BUG=6784
TEST=started and stopped http_server successfully.

Review URL: http://codereview.chromium.org/243022

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@28351 0039d316-1c4b-4281-b951-d872f2087c98