DevTools: move overview grid window curtains handling into said window

third_party/WebKit/Source/devtools/front_end/ui_lazy/timelineGrid.css

 /*
  * Copyright (c) 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.
  */
 
 .resources-dividers {
     position: absolute;
     left: 0;
     right: 0;
@@ skipping 47 matching lines @@
     right: 3px;
     font-size: 80%;
     white-space: nowrap;
     pointer-events: none;
 }
 
 .timeline-grid-header {
     height: 20px;
     pointer-events: none;
 }
-
-.timeline-curtain-left, .timeline-curtain-right {
-    background-color: hsla(0, 0%, 80%, 0.5);
-    position: absolute;
-    top: 0;
-    height: 100%;
-    z-index: 300;
-    pointer-events: none;
-    border: 1px none hsla(0, 0%, 70%, 0.5);
-}
-
-.timeline-curtain-left {
-    left: 0;
-    border-right-style: solid;
-}
-
-.timeline-curtain-right {
-    right: 0;
-    border-left-style: solid;
-}