Unprefix flexbox properties in various CSS files

Source/core/css/mediaControlsAndroid.css

 /*
  * Copyright (C) 2012 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 19 matching lines @@
 
 audio::-webkit-media-controls-enclosure, video::-webkit-media-controls-enclosure {
     height: 35px;
 }
 
 audio::-webkit-media-controls-overlay-enclosure {
     display: none;
 }
 
 video::-webkit-media-controls-overlay-enclosure {
-    display: -webkit-flex;
+    display: flex;
     position: relative;
-    -webkit-flex-direction: column;
-    -webkit-justify-content: flex-end;
-    -webkit-align-items: center;
-    -webkit-flex: 1 1;
+    flex-direction: column;
+    justify-content: flex-end;
+    align-items: center;
+    flex: 1 1;
     width: 100%;
     max-width: 800px;
     text-indent: 0;
     box-sizing: border-box;
     overflow: hidden;
 }
 
 audio::-webkit-media-controls-panel, video::-webkit-media-controls-panel {
     height: 35px;
     border-radius: 0;
 }
 
 audio::-webkit-media-controls-mute-button, video::-webkit-media-controls-mute-button {
     display: none;
 }
 
 video::-webkit-media-controls-overlay-play-button {
     -webkit-appearance: media-overlay-play-button;
-    display: -webkit-flex;
+    display: flex;
     position: absolute;
     top: 50%;
     left: 50%;
     margin-left: -40px;
     margin-top: -40px;
     border: none;
     box-sizing: border-box;
     background-color: transparent;
     width: 80px;
     height: 80px;
@@ skipping 25 matching lines @@
 
 audio::-webkit-media-controls-fullscreen-button {
     display: none;
 }
 
 audio::-webkit-media-controls-toggle-closed-captions-button, video::-webkit-media-controls-toggle-closed-captions-button {
     width: 35px;
     height: 35px;
     line-height: 35px;
 }