Bug 1648064 - Disable the URL in the header in printpreview_helper. r=nordzilla

It seems win7 sometimes displaces it slightly. We've hit this in the
past and it's not the point of the test so just get rid of it.

Differential Revision: https://phabricator.services.mozilla.com/D84597

layout/base/tests/chrome/printpreview_helper.xhtml

@@ -63,6 +63,8 @@ function printpreview(hasMozPrintCallback) {
                        .getService(Ci.nsIPrintSettingsService).globalPrintSettings;
   settings.showPrintProgress = false;
   settings.printBGColors = true;
+  settings.headerStrLeft = "";
+  settings.headerStrRight = "";
   var before = 0;
   var after = 0;
   function beforeprint() { ++before; }