Bug 841413 - Part 1: Convert test_privbrowsing_perwindowpb.html into a chrome test; r=jdm

--HG--
extra : rebase_source : 882e654c1cc68d50ae8b066341b1d2b3f785acfd
Bug 841413 - Part 2: Don't call enablePrivilege needlessly in localstorage Private Browsing tests; r=jdm

--HG--
extra : rebase_source : ae0c97cf0145d356ac2c520645c6bbc45c7de0ee