Switch tracing to new flexbox.

tracing/tracing/ui/extras/about_tracing/common.css

 /* Copyright (c) 2012 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,
 body {
   height: 100%;
 }
 
 body {
-  -webkit-flex-direction: column;
-  display: -webkit-flex;
+  flex-direction: column;
+  display: flex;
   margin: 0;
   padding: 0;
 }
 
 body > x-profiling-view {
-  -webkit-flex: 1 1 auto;
+  flex: 1 1 auto;
   min-height: 0;
 }
 
 body > x-profiling-view > x-timeline-view:focus {
   outline: 0
 }