Added showing slow scroll rectangles in Layers panel.

Source/devtools/front_end/layersPanel.css

Index: Source/devtools/front_end/layersPanel.css
diff --git a/Source/devtools/front_end/layersPanel.css b/Source/devtools/front_end/layersPanel.css
index c558fb809442529e34d0383bf33061bcd14d5895..3a8b46885039806551209a8b348e689139ff0f97 100644
--- a/Source/devtools/front_end/layersPanel.css
+++ b/Source/devtools/front_end/layersPanel.css
@@ -131,6 +131,12 @@
     border-style: solid;
 }
 
+.scroll-rect {
+    border: solid rgb(178, 0, 0);
+    border-width: inherit;
+    background-color: rgba(178, 0, 0, 0.2);
+}
+
 .layer-details-view .empty-view {
     font-size: 16px;
 }