Bug 1318273 - Improve the use of SpecialPowers.pushPrefEnv() - part 2, r=qdot

browser/components/sessionstore/test/head.js

browser/modules/test/browser_ProcessHangNotifications.js

dom/security/test/general/test_block_script_wrong_mime.html

...workers/test/serviceworkers/test_installation_simple.html

...onents/places/tests/browser/browser_visituri_nohistory.js

...nt/tests/browser/browser_bug295977_autoscroll_overflow.js