[AppRemoting] Implement ConnectionDroppedDialog.

remoting/webapp/app_remoting/html/ar_dialog.css

 /* Copyright 2014 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #idle-dialog {
   position: absolute;
   top: 100px;
   width: 100%;
 }
 
-#idle-dialog .kd-modaldialog,
-#auth-dialog .kd-modaldialog {
+.app-remoting .kd-modaldialog {
   /*
    * kd-modaldialog uses outline, which doesn't affect the bounding box, and so
    * doesn't work well when adjusting the window shape.
    */
   border: 1px solid gray;
 }
+
+#connection-dropped-dialog {
+  margin: 0px;
+  position: absolute;
+}