Remove IPC::BrokerableAttachment.

ipc/ipc_mojo_handle_attachment.cc

Index: ipc/ipc_mojo_handle_attachment.cc
diff --git a/ipc/ipc_mojo_handle_attachment.cc b/ipc/ipc_mojo_handle_attachment.cc
index 819a12b890ad1c3da3857f3f36923c16854576e4..e3421c3e88bc31aa78b4d0b51427cb4948bb5d36 100644
--- a/ipc/ipc_mojo_handle_attachment.cc
+++ b/ipc/ipc_mojo_handle_attachment.cc
@@ -18,16 +18,9 @@ MojoHandleAttachment::~MojoHandleAttachment() {
 }
 
 MessageAttachment::Type MojoHandleAttachment::GetType() const {
-  return TYPE_MOJO_HANDLE;
+  return Type::MOJO_HANDLE;
 }
 
-#if defined(OS_POSIX)
-base::PlatformFile MojoHandleAttachment::TakePlatformFile() {
-  NOTREACHED();
-  return base::kInvalidPlatformFile;
-}
-#endif  // OS_POSIX
-
 mojo::ScopedHandle MojoHandleAttachment::TakeHandle() {
   return std::move(handle_);
 }