ipc: Add methods for AttachmentBroker nonce serialization.

ipc/brokerable_attachment.h

Index: ipc/brokerable_attachment.h
diff --git a/ipc/brokerable_attachment.h b/ipc/brokerable_attachment.h
index 452d1297aa9df2cfdaa173652131b5f7a944d407..9c2bbbd69ec444ac0770a1df93cf7ae34110be20 100644
--- a/ipc/brokerable_attachment.h
+++ b/ipc/brokerable_attachment.h
@@ -22,6 +22,15 @@ class IPC_EXPORT BrokerableAttachment : public MessageAttachment {
   struct IPC_EXPORT AttachmentId {
     uint8_t nonce[kNonceSize];
 
+    // Default constructor returns an unguessable random nonce.
+    AttachmentId();
+
+    // Constructs an AttachmentId from a buffer.
+    AttachmentId(const char* start_address, size_t size);
+
+    // Writes the nonce into a buffer.
+    void SerializeToBuffer(char* start_address, size_t size);
+
     bool operator==(const AttachmentId& rhs) const {
       for (size_t i = 0; i < kNonceSize; ++i) {
         if (nonce[i] != rhs.nonce[i])