window.sessionStorage throws an exception when cookies are disabled in some browsers

Bug: Accessing window.sessionStorage throws an exception in some browsers (including Chrome) when cookies/localStorage are explicitly disabled. This causes history.js to completely die because one of the very first things it does is to check sessionStorage availability outside of a try/catch.

To reproduce:
1. In Chrome, disable cookies.
2. Hit any page with history.js
3. Note that an exception is thrown when accessing window.sessionStorage