Remove the -webkit-fullscreen-volume-* pseudos, they're dead code.

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 330 matching lines @@
     width: 32px;
     height: 32px;
     padding: 0px;
     border-width: 0px;
     margin-left: 0px;
     margin-right: 0px;
     background-color: initial;
     color: inherit;
 }
 
-audio::-webkit-media-controls-fullscreen-volume-slider, video::-webkit-media-controls-fullscreen-volume-slider {
-    display: none;
-}
-
-audio::-webkit-media-controls-fullscreen-volume-min-button, video::-webkit-media-controls-fullscreen-volume-min-button {
-    display: none;
-}
-
-audio::-webkit-media-controls-fullscreen-volume-max-button, video::-webkit-media-controls-fullscreen-volume-max-button {
-    display: none;
-}
-
 video::-internal-media-controls-text-track-list, video::-internal-media-controls-overflow-menu-list, audio::-internal-media-controls-overflow-menu-list {
     position: absolute;
     bottom: 4px;
     right: 4px;
     background-color: #fafafa;
     max-width: 50%;
     max-height: 250px;
     min-width: 150px;
     overflow-x: hidden;
     overflow-y: auto;
@@ skipping 153 matching lines @@
     font-weight: bold;
 }
 
 video::cue(u) {
     text-decoration: underline;
 }
 
 video::cue(i) {
     font-style: italic;
 }