DevTools: Support popover on timeline overview.

Source/devtools/front_end/ui_lazy/overviewGrid.css

 /*
  * Copyright (c) 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.
  */
 
 .overview-grid-window-selector {
     position: absolute;
     top: 0;
     bottom: 0;
@@ skipping 43 matching lines @@
     z-index: 500;
     border-radius: 2px;
     box-shadow: white 1px 0 0, white -1px 0 0, white 0 1px 0, white 0 -1px 0;
 }
 
 .overview-grid-window-resizer-right {
     margin-left: -3px;
     margin-right: -2px;
 }
 
-.overview-grid-window-area {
-    position: absolute;
-    top: 20px;
-    bottom: 0;
-    left: 0;
-    right: 0;
-    z-index: 250;
-}
-
 .overview-grid-current-position {
     position: absolute;
     top: 0;
     bottom: 0;
     width: 2px;
     background-color: hsla(220, 95%, 50%, 0.7);
     z-index: 500;
     pointer-events: none;
     visibility: hidden;
 }