[DevTools] Make UI.GlassPane position contentElement for different overlay controls.

third_party/WebKit/Source/devtools/front_end/ui/inspectorCommon.css

 /*
  * Copyright 2015 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.
  */
 
 * {
     /* This is required for correct sizing of flex items because we rely
      * on an old version of the flexbox spec.
      * Longer-term we should remove this, see crbug.com/473625 */
@@ skipping 368 matching lines @@
     background-color: white;
 }
 
 .force-white-icons [is=ui-icon].spritesheet-resourceicons, [is=ui-icon].force-white-icons.spritesheet-resourceicons {
     -webkit-mask-image: -webkit-image-set(url(Images/resourceGlyphs.png) 1x, url(Images/resourceGlyphs_2x.png) 2x);
     -webkit-mask-size: 140px 20px;
     background-image: unset;
     background-size: unset;
     background: unset;
     background-color: white;
-}
+}
+
+.glass-pane {
+    position: absolute;
+    top: 0;
+    bottom: 0;
+    left: 0;
+    right: 0;
+    overflow: hidden;
+}
+
+.glass-pane-content {
+    display: flex;
+}