Unprefix flexbox properties in various CSS files

Source/core/css/fullscreen.css

 :-webkit-full-screen {
     background-color: white;
     z-index: 2147483647 !important;
 }
 
 :root:-webkit-full-screen-document:not(:-webkit-full-screen), :root:-webkit-full-screen-ancestor {
     overflow: hidden !important;
 }
 
 :-webkit-full-screen-ancestor:not(iframe) {
     z-index: auto !important;
     position: static !important;
     opacity: 1 !important;
     -webkit-transform: none !important;
     -webkit-mask: none !important;
     clip: none !important;
     -webkit-filter: none !important;
     transition: none !important;
     -webkit-box-reflect: none !important;
     -webkit-perspective: none !important;
     -webkit-transform-style: flat !important;
 }
 
 video:-webkit-full-screen, audio:-webkit-full-screen {
     background-color: transparent !important;
     position: static !important;
     margin: 0 !important;
     height: 100% !important;
     width: 100% !important;
-    -webkit-flex: 1 !important;
+    flex: 1 !important;
     display: block !important;
 }
 
 img:-webkit-full-screen {
     width: auto;
     height: 100%;
     max-width: 100%;
 }
 
 iframe:-webkit-full-screen {
     margin: 0 !important;
     padding: 0 !important;
     border: 0 !important;
     position: fixed !important;
     height: 100% !important;
     width: 100% !important;
     left: 0 !important;
     top: 0 !important;
 }