Document why we choose SOCK_SEQPACKET.

chrome/browser/zygote_host_linux.cc

Index: chrome/browser/zygote_host_linux.cc
===================================================================
--- chrome/browser/zygote_host_linux.cc	(revision 26988)
+++ chrome/browser/zygote_host_linux.cc	(working copy)
@@ -54,7 +54,14 @@
                                  switches::kZygoteProcess);
 
   int fds[2];
+
+  // SOCK_SEQPACKET is used rather than SOCK_DGRAM because SOCK_DGRAM
+  // allows sendto()/sendmsg() to send to arbitrary addresses, and the
+  // SUID sandbox cannot prevent that. Apart from this security
+  // problem, SOCK_DGRAM also has the correct semantics for this
+  // socket pair.
   CHECK(socketpair(PF_UNIX, SOCK_SEQPACKET, 0, fds) == 0);
+
   base::file_handle_mapping_vector fds_to_map;
   fds_to_map.push_back(std::make_pair(fds[1], 3));