POSIX: Rewrite IPC's interaction with FileDescriptor

base/file_descriptor_posix.h

Index: base/file_descriptor_posix.h
diff --git a/base/file_descriptor_posix.h b/base/file_descriptor_posix.h
index 8ebc5be59a96f2a73d8dea658900789cc59f0e8e..0bcd4b6c6955663a83a6727be2bfeab237d81ebd 100644
--- a/base/file_descriptor_posix.h
+++ b/base/file_descriptor_posix.h
@@ -10,6 +10,10 @@ namespace base {
 // -----------------------------------------------------------------------------
 // We introduct a special structure for file descriptors in order that we are
 // able to use template specialisation to special-case their handling.
+//
+// WARNING: (Chromium only) There are subtleties to consider if serialising
+// these objects over IPC. See comments in chrome/common/ipc_message_utils.h
+// above the template specialisation for this structure.
 // -----------------------------------------------------------------------------
 struct FileDescriptor {
   FileDescriptor()