Fix dialog screen position.

remoting/webapp/window_frame.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.
  */
 
+.window-body {
+  position: relative;
+}
+
 html.apps-v2,
 html.apps-v2 body {
   height: 100%;
   width: 100%;
 }
 
 html.apps-v2 body:not(.fullscreen) {
   border: 1px solid gray;  /* This is the window border. */
 }
 
@@ skipping 166 matching lines @@
   box-shadow: 1px 1px 10px rgba(0, 0, 0, 0.5);
 }
 
 .fullscreen .title-bar.opened .window-controls-stub {
   background-color: #a6a6a6;
 }
 
 body.fullscreen .window-fullscreen {
   display: none;
 }