Add a SetDoNotFragment() method to DatagramSocket, and call this for

net/udp/udp_socket_posix.h

Index: net/udp/udp_socket_posix.h
diff --git a/net/udp/udp_socket_posix.h b/net/udp/udp_socket_posix.h
index 305c435334ae0c4f27de4f29b72b66671a92207a..e0e57debc2cc88d81e637606993c28b5cfd5a2d0 100644
--- a/net/udp/udp_socket_posix.h
+++ b/net/udp/udp_socket_posix.h
@@ -119,6 +119,13 @@ class NET_EXPORT UDPSocketPosix : public base::NonThreadSafe {
   // Returns a net error code.
   int SetSendBufferSize(int32_t size);
 
+  // If supported by the platform, configures the socket to set the DF
+  // (Do not Fragment) bit on packets, which prevents routers from fragmenting
+  // them. Also, for IPv6, it prevents hosts from fragmenting them.
+  // Returns a return a network error code if there was a problem, but the
+  // socket will still be usable. Can not return ERR_IO_PENDING.
+  int SetDoNotFragment();
+
   // Returns true if the socket is already connected or bound.
   bool is_connected() const { return is_connected_; }