Taken from Fedora's chromium SPEC file:

To use round with gcc, you need to #include <cmath>

diff -up chromium-65.0.3325.146/third_party/webrtc/p2p/base/port.cc.gcc-round-fix chromium-65.0.3325.146/third_party/webrtc/p2p/base/port.cc
--- chromium-65.0.3325.146/third_party/webrtc/p2p/base/port.cc.gcc-round-fix	2018-03-07 10:57:11.284376048 -0500
+++ chromium-65.0.3325.146/third_party/webrtc/p2p/base/port.cc	2018-03-07 10:58:16.590742636 -0500
@@ -10,7 +10,7 @@
 
 #include "p2p/base/port.h"
 
-#include <math.h>
+#include <cmath>
 
 #include <algorithm>
 #include <utility>