Switch tracing to new flexbox.

tracing/tracing/ui/timeline_view_help_overlay.html

Index: tracing/tracing/ui/timeline_view_help_overlay.html
diff --git a/tracing/tracing/ui/timeline_view_help_overlay.html b/tracing/tracing/ui/timeline_view_help_overlay.html
index 667b1a46525cc6f7db14a020fa66aa4916044436..48c2e72df76971fca2d8978cf78dd0c8414f8eee 100644
--- a/tracing/tracing/ui/timeline_view_help_overlay.html
+++ b/tracing/tracing/ui/timeline_view_help_overlay.html
@@ -12,9 +12,9 @@ found in the LICENSE file.
   <template>
     <style>
     :host {
-      -webkit-flex: 1 1 auto;
-      -webkit-flex-direction: row;
-      display: -webkit-flex;
+      flex: 1 1 auto;
+      flex-direction: row;
+      display: flex;
       width: 700px;
     }
     .column {
@@ -32,9 +32,9 @@ found in the LICENSE file.
       margin-top: 10px;
     }
     .pair {
-      -webkit-flex: 1 1 auto;
-      -webkit-flex-direction: row;
-      display: -webkit-flex;
+      flex: 1 1 auto;
+      flex-direction: row;
+      display: flex;
     }
     .command {
       font-family: monospace;