Switch tracing to new flexbox.

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

Index: tracing/tracing/ui/extras/about_tracing/common.css
diff --git a/tracing/tracing/ui/extras/about_tracing/common.css b/tracing/tracing/ui/extras/about_tracing/common.css
index 1ffb5f72e74ded35a77b0919014df05012422ae5..3db21b67ffa202d19234922455625f4a7524c098 100644
--- a/tracing/tracing/ui/extras/about_tracing/common.css
+++ b/tracing/tracing/ui/extras/about_tracing/common.css
@@ -8,14 +8,14 @@ body {
 }
 
 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;
 }