VrShell: Allow native control of UI element opacity.

chrome/browser/resources/vr_shell/vr_shell_ui.css

 /* Copyright 2016 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. */
 
 html {
   background-color: rgba(255, 255, 255, 0);
 }
 
 #ui {
   left: 0;
@@ skipping 110 matching lines @@
 
 #omnibox-container {
   --tranX: 0;
   --tranY: -0.65;
   --tranZ: -1.2;
 }
 
 #omnibox-border {
   --fadeTimeMs: 500;
   --fadeYOffset: -0.1;
+  --opacity: 0.9;
   --statusBarColor: rgb(66, 133, 244);
   background-color: #ececec;
   border-radius: 200px;
-  opacity: 0.9;
   padding: 6px;
-  transition: opacity 500ms ease;
-}
-
-#omnibox-border.hidden {
-  opacity: 0;
 }
 
 #omnibox {
   align-items: center;
   background-color: #ececec;
   border-radius: 200px;
   box-sizing: border-box;
   display: flex;
   height: 104px;
   justify-content: center;
@@ skipping 36 matching lines @@
   color: #252525;
   font-size: 34px;
   overflow: hidden;
   white-space: nowrap;
 }
 
 #omnibox #path {
   color: #868686;
 }