[mojo-edk] Support transferring null Mach ports.

mojo/edk/system/channel.h

Index: mojo/edk/system/channel.h
diff --git a/mojo/edk/system/channel.h b/mojo/edk/system/channel.h
index 9932c9c71fc0d1695dab89bc6d40491adb70a0a4..1d384a978b631b6bd61e44345f5d26c44f14b7e6 100644
--- a/mojo/edk/system/channel.h
+++ b/mojo/edk/system/channel.h
@@ -76,6 +76,13 @@ class Channel : public base::RefCountedThreadSafe<Channel> {
     };
     static_assert(sizeof(MachPortsEntry) == 6,
                   "sizeof(MachPortsEntry) must be 6 bytes");
+
+    struct MachPortsExtraHeader {
+      uint16_t num_ports;
+      MachPortsEntry entries[0];
+    };
+    static_assert(sizeof(MachPortsExtraHeader) == 2,
+                  "sizeof(MachPortsExtraHeader) must be 2 bytes");
 #endif
 #pragma pack(pop)
 
@@ -155,7 +162,7 @@ class Channel : public base::RefCountedThreadSafe<Channel> {
 
 #if defined(OS_MACOSX) && !defined(OS_IOS)
     // On OSX, handles are serialised into the extra header section.
-    MachPortsEntry* mach_ports_ = nullptr;
+    MachPortsExtraHeader* mach_ports_header_ = nullptr;
 #endif
 
     DISALLOW_COPY_AND_ASSIGN(Message);