Bug 1393424 - Don't try to autoscroll in Print Preview mode. r=kats

It doesn't work properly anyways due to a more general coordinate-system
problem on the Print Preview page (see bug 1393494).

MozReview-Commit-ID: D4i2H8z5mtW

toolkit/content/browser-content.js

@@ -139,6 +139,12 @@ var ClickEventHandler = {
     if (!this._scrollable)
       return;
 
+    // In some configurations like Print Preview, content.performance
+    // (which we use below) is null. Autoscrolling is broken in Print
+    // Preview anyways (see bug 1393494), so just don't start it at all.
+    if (!content.performance)
+      return;
+
     let domUtils = content.QueryInterface(Ci.nsIInterfaceRequestor)
                           .getInterface(Ci.nsIDOMWindowUtils);
     let scrollable = this._scrollable;