The setter changes per-thread data - for the DOM/UI thread, this means that we can set the JS Context to one with DOM magic. This magic allows some DOM JS <-> XPConnect JS conversions to succeed, and in particular allows creation of a DOM window from within a JS component. Unblocks nsbeta2 work by Vishy and Pavlov. r=mccabe,vishy. a=brendan,sleestack. git-svn-id: svn://10.0.0.236/trunk@70754 18797224-902f-48f8-a5cc-f745e15eee43