Add server base URL parameter to policy_testserver.py

This allows configurations where the test server runs behind a reverse
proxy and needs to return URLs that point at the proxy.

BUG=None

Review URL: https://codereview.chromium.org/25262002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234801 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234801