Bug 1132566 - Enable test in e10s: browser_privatebrowsing_sidebar.js; r=billm

This test appeared to run fine under e10s, so after running 30 times
with "mochitest-browser --e10s --run-until-failure" I've re-enabled it.

--HG--
extra : rebase_source : 58234a0d59ae8daeabfd392a0e54fdb18246dc86

browser/components/privatebrowsing/test/browser/browser.ini

@@ -43,7 +43,6 @@ skip-if = e10s # Bug 1139953 - Accept cookie dialog shown in private window when
 [browser_privatebrowsing_protocolhandler.js]
 skip-if = e10s # Bug 940206 -  nsIWebContentHandlerRegistrar::registerProtocolHandler doesn't work in e10s
 [browser_privatebrowsing_sidebar.js]
-skip-if = e10s
 [browser_privatebrowsing_theming.js]
 skip-if = e10s
 [browser_privatebrowsing_ui.js]