Revert of Revert of Make SharedMemoryHandle a class on Mac.

ipc/ipc_message_utils.h

Index: ipc/ipc_message_utils.h
diff --git a/ipc/ipc_message_utils.h b/ipc/ipc_message_utils.h
index 94651419be23314484f6fe6b91a8d2fc643c1e3e..96bb6895abb3f08c25dca3af78c7f8a26619ce38 100644
--- a/ipc/ipc_message_utils.h
+++ b/ipc/ipc_message_utils.h
@@ -57,6 +57,10 @@
 class TimeTicks;
 class TraceTicks;
 struct FileDescriptor;
+
+#if defined(OS_MACOSX) && !defined(OS_IOS)
+class SharedMemoryHandle;
+#endif  // defined(OS_MACOSX) && !defined(OS_IOS)
 }
 
 namespace IPC {
@@ -467,6 +471,16 @@
 };
 #endif  // defined(OS_POSIX)
 
+#if defined(OS_MACOSX) && !defined(OS_IOS)
+template <>
+struct IPC_EXPORT ParamTraits<base::SharedMemoryHandle> {
+  typedef base::SharedMemoryHandle param_type;
+  static void Write(Message* m, const param_type& p);
+  static bool Read(const Message* m, base::PickleIterator* iter, param_type* r);
+  static void Log(const param_type& p, std::string* l);
+};
+#endif
+
 template <>
 struct IPC_EXPORT ParamTraits<base::FilePath> {
   typedef base::FilePath param_type;