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

--HG--
rename : dom/plugins/test/mochitest/test_privatemode.xul => dom/plugins/test/mochitest/test_privatemode_perwindowpb.xul
extra : transplant_source : %94A%27%1B%DBEL%10-%C1%7B1%94%AF1%F4%29%B9%D6I