Bug 806723 - Port plugin test_privatemode.xul to the new per-tab PB APIs r=jdm

--HG--
rename : dom/plugins/test/mochitest/test_privatemode.xul => dom/plugins/test/mochitest/test_privatemode_perwindowpb.xul
extra : rebase_source : 482bddf5f3af6be0aa92ef53783e2153860f5478