A session is started when HttpRequest.getSession() is called. The
session will then be added to the session manager. The session manager
then controls timeouts.
The session id is create by using the a crypto-random sequence of 16
bytes. These are extracted by using /dev/urandom on posix and
CryptGenRandom on windows.
BUG=
Review URL: https://codereview.chromium.org//11175030
git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@13870 260f80e4-7a28-3924-810f-c04153c831b5