Re-land: [mojo-edk] Serialise windows handles into an "extra header" section.

mojo/edk/system/channel_win.cc

Index: mojo/edk/system/channel_win.cc
diff --git a/mojo/edk/system/channel_win.cc b/mojo/edk/system/channel_win.cc
index 367883540c2e5b64361fb4230ac469e08ed71388..1eeb2f644fe3ace5f2acb1f354f35de61d7c787a 100644
--- a/mojo/edk/system/channel_win.cc
+++ b/mojo/edk/system/channel_win.cc
@@ -121,17 +121,15 @@ class ChannelWin : public Channel,
 
   ScopedPlatformHandleVectorPtr GetReadPlatformHandles(
       size_t num_handles,
-      void** payload,
-      size_t* payload_size) override {
+      const void* extra_header,
+      size_t extra_header_size) override {
     size_t handles_size = sizeof(PlatformHandle) * num_handles;
-    if (handles_size > *payload_size)
+    if (handles_size > extra_header_size)
       return nullptr;
 
-    *payload_size -= handles_size;
     ScopedPlatformHandleVectorPtr handles(
         new PlatformHandleVector(num_handles));
-    memcpy(handles->data(),
-           static_cast<const char*>(*payload) + *payload_size, handles_size);
+    memcpy(handles->data(), extra_header, handles_size);
     return handles;
   }