Generate param traits size methods for IPC files in content/ (and traits it depends on).

ipc/mach_port_mac.h

Index: ipc/mach_port_mac.h
diff --git a/ipc/mach_port_mac.h b/ipc/mach_port_mac.h
index b95a37b47d036786f592aed06fa4177d98e48b41..f8511344f6b8d24e7d0617183d1e0e51c0438647 100644
--- a/ipc/mach_port_mac.h
+++ b/ipc/mach_port_mac.h
@@ -71,6 +71,7 @@ class IPC_EXPORT MachPortMac {
 template <>
 struct IPC_EXPORT ParamTraits<MachPortMac> {
   typedef MachPortMac param_type;
+  static void GetSize(base::PickleSizer* s, const param_type& p);
   static void Write(base::Pickle* m, const param_type& p);
   static bool Read(const base::Pickle* m,
                    base::PickleIterator* iter,