Update sync API to support attachments.

sync/api/attachments/attachment_id.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SYNC_API_ATTACHMENTS_ATTACHMENT_ID_H_
 #define SYNC_API_ATTACHMENTS_ATTACHMENT_ID_H_
 
 #include <string>
 #include <vector>
 
 #include "sync/base/sync_export.h"
+#include "sync/internal_api/public/util/immutable.h"
+
+namespace sync_pb {
+class AttachmentIdProto;
+}  // namespace sync_pb
 
 namespace syncer {
 
 // Uniquely identifies an attachment.
 //
 // Two attachments with equal (operator==) AttachmentIds are considered
 // equivalent.
 class SYNC_EXPORT AttachmentId {
  public:
   ~AttachmentId();
 
   // Default copy and assignment are welcome.
 
   bool operator==(const AttachmentId& other) const;
 
   bool operator!=(const AttachmentId& other) const;
 
   // Needed for using AttachmentId as key in std::map.
   bool operator<(const AttachmentId& other) const;
 
-
   // Creates a unique attachment id.
   static AttachmentId Create();
 
+  // Creates an attachment id from an initialized proto.
+  static AttachmentId CreateFromProto(const sync_pb::AttachmentIdProto& proto);
+
+  const sync_pb::AttachmentIdProto& GetProto() const;
+
  private:
-  std::string unique_id_;
+  // Necessary since we forward-declare sync_pb::AttachmentIdProto; see comments
+  // in immutable.h.
+  struct ImmutableAttachmentIdProtoTraits {
+    typedef sync_pb::AttachmentIdProto* Wrapper;
+    static void InitializeWrapper(Wrapper* wrapper);
+    static void DestroyWrapper(Wrapper* wrapper);
+    static const sync_pb::AttachmentIdProto& Unwrap(const Wrapper& wrapper);
+    static sync_pb::AttachmentIdProto* UnwrapMutable(Wrapper* wrapper);
+    static void Swap(sync_pb::AttachmentIdProto* t1,
+                     sync_pb::AttachmentIdProto* t2);
+  };
 
-  AttachmentId(const std::string& unique_id);
+  typedef Immutable<sync_pb::AttachmentIdProto,
+                    ImmutableAttachmentIdProtoTraits>
+      ImmutableAttachmentIdProto;
+
+  ImmutableAttachmentIdProto proto_;
+
+  AttachmentId(sync_pb::AttachmentIdProto* proto);
 };
 
 typedef std::vector<AttachmentId> AttachmentIdList;
 
 }  // namespace syncer
 
 #endif  // SYNC_API_ATTACHMENTS_ATTACHMENT_ID_H_