[Sync] Add size and crc32c to AttachmentId.

sync/api/attachments/attachment_id.h

Index: sync/api/attachments/attachment_id.h
diff --git a/sync/api/attachments/attachment_id.h b/sync/api/attachments/attachment_id.h
index 6ce331a98aa55091a0eae4f4b391113d888bc245..1c53355f8c476acb2fd81267eedd378c7582c9ea 100644
--- a/sync/api/attachments/attachment_id.h
+++ b/sync/api/attachments/attachment_id.h
@@ -35,14 +35,24 @@ class SYNC_EXPORT AttachmentId {
   // Needed for using AttachmentId as key in std::map.
   bool operator<(const AttachmentId& other) const;
 
-  // Creates a unique attachment id.
-  static AttachmentId Create();
+  // Creates a unique id for an attachment.
+  //
+  // |size| is the attachment's size in bytes.
+  //
+  // |crc32c| is the attachment's crc32c.
+  static AttachmentId Create(size_t size, uint32_t crc32c);
 
   // Creates an attachment id from an initialized proto.
   static AttachmentId CreateFromProto(const sync_pb::AttachmentIdProto& proto);
 
   const sync_pb::AttachmentIdProto& GetProto() const;
 
+  // Returns the size (in bytes) the attachment.
+  size_t GetSize() const;
+
+  // Returns the crc32c the attachment.
+  uint32_t GetCrc32c() const;
+
  private:
   // Necessary since we forward-declare sync_pb::AttachmentIdProto; see comments
   // in immutable.h.