Revert of [mojo-edk] Serialise windows handles into an "extra header" section.

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 43ed579fe1019e13cca1e518c977baba4c4ef005..bfc1232be2053f525fd28fc5069cbabc2b84951b 100644
--- a/mojo/edk/system/channel.h
+++ b/mojo/edk/system/channel.h
@@ -29,7 +29,6 @@
 
   // A message to be written to a channel.
   struct Message {
-#pragma pack(push, 1)
     struct Header {
       enum class MessageType : uint16_t {
         // A normal message.
@@ -45,25 +44,17 @@
       // Message size in bytes, including the header.
       uint32_t num_bytes;
 
-      // Total size of header, including extra header data (i.e. HANDLEs on
-      // windows).
-      uint16_t num_header_bytes;
-
-      // Number of attached handles. May be less than the reserved handle
-      // storage size in this message on platforms that serialise handles as
-      // data (i.e. HANDLEs on Windows, Mach ports on OSX).
+      // Number of attached handles.
       uint16_t num_handles;
 
       MessageType message_type;
-
-      char padding[6];
     };
-#pragma pack(pop)
 
     // Allocates and owns a buffer for message data with enough capacity for
-    // |payload_size| bytes plus a header, plus |max_handles| platform handles.
+    // |payload_size| bytes plus a header. Takes ownership of |handles|, which
+    // may be null.
     Message(size_t payload_size,
-            size_t max_handles,
+            size_t num_handles,
             Header::MessageType message_type = Header::MessageType::NORMAL);
 
     ~Message();
@@ -74,22 +65,15 @@
     const void* data() const { return data_; }
     size_t data_num_bytes() const { return size_; }
 
-    const void* extra_header() const { return data_ + sizeof(Header); }
-    void* mutable_extra_header() { return data_ + sizeof(Header); }
-    size_t extra_header_size() const {
-      return header_->num_header_bytes - sizeof(Header);
+    void* mutable_payload() { return static_cast<void*>(header_ + 1); }
+    const void* payload() const {
+      return static_cast<const void*>(header_ + 1);
     }
-
-    void* mutable_payload() { return data_ + header_->num_header_bytes; }
-    const void* payload() const { return data_ + header_->num_header_bytes; }
     size_t payload_size() const;
 
     size_t num_handles() const { return header_->num_handles; }
     bool has_handles() const { return header_->num_handles > 0; }
     PlatformHandle* handles();
-#if defined(OS_MACOSX) && !defined(OS_IOS)
-    bool has_mach_ports() const;
-#endif
 
     // Note: SetHandles() and TakeHandles() invalidate any previous value of
     // handles().
@@ -110,7 +94,6 @@
 
    private:
     size_t size_;
-    size_t max_handles_;
     char* data_;
     Header* header_;
 
@@ -192,16 +175,17 @@
   // OK to call this synchronously from any public interface methods.
   void OnError();
 
-  // Retrieves the set of platform handles read for a given message.
-  // |extra_header| and |extra_header_size| correspond to the extra header data.
-  // Depending on the Channel implementation, this body may encode platform
-  // handles, or handles may be stored and managed elsewhere by the
-  // implementation.  If |num_handles| handles cannot be returned, this must
-  // return null.
+  // Retrieves the set of platform handles read for a given message. |payload|
+  // and |payload_size| correspond to the full message body. Depending on
+  // the Channel implementation, this body may encode platform handles, or
+  // handles may be stored and managed elsewhere by the implementation.
+  // If |num_handles| handles cannot be returned, this must return null.
+  // The implementation may also adjust the values of |*payload| and/or
+  // |*payload_size| to hide handle data from the user.
   virtual ScopedPlatformHandleVectorPtr GetReadPlatformHandles(
       size_t num_handles,
-      const void* extra_header,
-      size_t extra_header_size) = 0;
+      void** payload,
+      size_t* payload_size) = 0;
 
   virtual void OnControlMessage(Message::Header::MessageType message_type,
                                 const void* payload,