HTML5 Storage API in GWT.

This change adds the HTML5 local and session storage APIs, and a Map interface
backed by storage. This is a contribution from an external project,
the gwt-mobile-webkit project, and is a copy of issue#1290802 with some additional changes.

Review at http://gwt-code-reviews.appspot.com/1374803

Review by: jlabanca@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9818 8db76d5a-ed1c-0410-87a9-c151d255dfc7