[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 350 matching lines @@
 }
 
 .force-white-icons [is=ui-icon].spritesheet-largeicons, [is=ui-icon].force-white-icons.spritesheet-largeicons {
     -webkit-mask-image: -webkit-image-set(url(Images/toolbarButtonGlyphs.png) 1x, url(Images/toolbarButtonGlyphs_2x.png) 2x);
     -webkit-mask-size: 352px 168px;
     background-image: unset;
     background-size: unset;
     background: unset;
     background-color: white;
 }
+
+.modal-overlay {
+    position: absolute;
+    display: flex;
+    justify-content: flex-start;
+    align-items: stretch;
+}