Added showing slow scroll rectangles in Layers panel.

Source/devtools/front_end/layersPanel.css

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 113 matching lines @@
 
 .layer-container.hovered:not(.selected) {
     border-color: rgb(150, 150, 255);
 }
 
 .layer-container .paint-rect {
     position: absolute;
     border-style: solid;
 }
 
+.scroll-rect {
+    border: solid rgb(178, 0, 0);
+    border-width: inherit;
+    background-color: rgba(178, 0, 0, 0.4);
+}
+
 .layer-details-view .empty-view {
     font-size: 16px;
 }
 
 .layer-details-view table td {
     padding-left: 8px;
 }
 
 .layer-details-view table td:first-child {
     font-weight: bold;
@@ skipping 11 matching lines @@
 
 .paint-profiler-view .overview-grid-dividers-background,
 .paint-profiler-view .overview-grid-window {
     bottom: 0;
     height: auto;
 }
 
 .paint-profiler-view .overview-grid-window-resizer {
     z-index: 2000;
 }