Devtools: Replace split widget rAF animation with web animation

Source/devtools/front_end/ui/splitWidget.css

 /*
  * Copyright (C) 2011 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
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+:host {
+    overflow: hidden;
+}
+
 .shadow-split-widget {
     display: flex;
     overflow: hidden;
 }
 
 .shadow-split-widget-contents {
     display: flex;
     position: relative;
     flex-direction: column;
 }
@@ skipping 150 matching lines @@
 
 .shadow-split-widget button.bottom-sidebar-show-hide-button.toggled-show {
     -webkit-mask-position: -296px -76px; /* <| */
     transform: rotate(90deg);
 }
 
 .shadow-split-widget button.bottom-sidebar-show-hide-button.toggled-hide {
     -webkit-mask-position: -264px -76px; /* >| */
     transform: rotate(90deg);
 }