Bug 1357975 - Set pref browser.storageManager.enabled for storage browser tests, r=janv

dom/quota/test/browser_permissionsPrompt.html

@@ -13,7 +13,8 @@
         SpecialPowers.pushPrefEnv({
           "set": [["dom.storageManager.enabled", true],
                   ["dom.storageManager.prompt.testing", false],
-                  ["dom.storageManager.prompt.testing.allow", false]]
+                  ["dom.storageManager.prompt.testing.allow", false],
+                  ["browser.storageManager.enabled", true]]
         }, continueToNextStep);
         yield undefined;