Issue #2029 - Part 2: Protocol handling dialog should be dependent if not modal

Following the behavior of the download dialog, the content handling dialog should be `dependent` as well. This means that this dialog should close when the parent window is closed, is not visible on the taskbar, and stays in front of the parent window.

toolkit/mozapps/handling/nsContentDispatchChooser.js

@@ -66,6 +66,8 @@ nsContentDispatchChooser.prototype =
     var features = "chrome,dialog=yes,resizable,centerscreen";
     if (Services.prefs.getBoolPref("prompts.content_handling_dialog_modal.enabled")) {
       features += ",modal";
+    } else {
+      features += ",dependent";
     }
 
     var ww = Cc["@mozilla.org/embedcomp/window-watcher;1"].