Bug 817463 - Disable loading the homepage for per-window private browsing tests; r=jdm

DONTBUILD since this is NPOTB in global PB builds

browser/components/privatebrowsing/test/browser/perwindow/head.js

@@ -44,3 +44,12 @@ function clearHistory() {
   // simulate clearing the private data
   Services.obs.notifyObservers(null, "browser:purge-session-history", "");
 }
+
+function _initTest() {
+  // Don't use about:home as the homepage for new windows
+  Services.prefs.setIntPref("browser.startup.page", 0);
+  registerCleanupFunction(function() Services.prefs.clearUserPref("browser.startup.page"));
+}
+
+_initTest();
+