Backout 8a6de68c0feb (Fix for bug 571159 (Leak nsGlobalWindow with unknown-content-type dialog)) to fix orange.

--HG--
extra : rebase_source : b405b0ac1c54dd3af3141d8762b26e91e2e63a02

toolkit/mozapps/downloads/nsHelperAppDlg.js

@@ -929,7 +929,6 @@ nsUnknownContentTypeDialog.prototype = {
 
     // Unhook dialog from this object.
     this.mDialog.dialog = null;
-    this.mDialog = null;
 
     // Close up dialog by returning true.
     return true;
@@ -949,7 +948,6 @@ nsUnknownContentTypeDialog.prototype = {
 
     // Unhook dialog from this object.
     this.mDialog.dialog = null;
-    this.mDialog = null;
 
     // Close up dialog by returning true.
     return true;