IPC: Add attachment brokering support to the message header.

ipc/ipc_channel.cc

Index: ipc/ipc_channel.cc
diff --git a/ipc/ipc_channel.cc b/ipc/ipc_channel.cc
index ac09c5ab1ef1d00b31c4f3a2e7b0a260673dbfbe..89505cb5f76e40ebae0075197c95b3199a0f7fbf 100644
--- a/ipc/ipc_channel.cc
+++ b/ipc/ipc_channel.cc
@@ -41,4 +41,26 @@ std::string Channel::GenerateUniqueRandomChannelID() {
       base::RandInt(0, std::numeric_limits<int32>::max()));
 }
 
+Channel::OutputElement::OutputElement(Message* message)
+    : message_(message), buffer_(nullptr), length_(0) {}
+
+Channel::OutputElement::OutputElement(void* buffer, size_t length)
+    : message_(nullptr), buffer_(buffer), length_(length) {}
+
+Channel::OutputElement::~OutputElement() {
+  free(buffer_);
+}
+
+size_t Channel::OutputElement::size() const {
+  if (message_)
+    return message_->size();
+  return length_;
+}
+
+const void* Channel::OutputElement::data() const {
+  if (message_)
+    return message_->data();
+  return buffer_;
+}
+
 }  // namespace IPC