Enable smooth transition when show/hide media remoting interstitial.

third_party/WebKit/Source/core/css/mediaControls.css

 /*
  * Copyright (C) 2009 Apple Inc.  All rights reserved.
  * Copyright (C) 2015 Google Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 139 matching lines @@
     width: inherit;
     height: inherit;
     position: relative;
     direction: ltr;
     display: flex;
     flex-direction: column;
     justify-content: flex-end;
     align-items: center;
     font-size: 16px;
     background-color: black;
-    transition: opacity .2s ease-in-out;
+    opacity: 0;
+    transition: opacity .3s ease-in-out;
 }
 
 video::-internal-media-remoting-background-image {
     display: flex;
     position: absolute;
     margin: 0;
     top: 0px;
     left: 0px;
     width: 100%;
     height: 100%;
@@ skipping 430 matching lines @@
     font-weight: bold;
 }
 
 video::cue(u) {
     text-decoration: underline;
 }
 
 video::cue(i) {
     font-style: italic;
 }