Keep track of which attachments are referenced by which sync entries.

sync/api/attachments/attachment_service.h

Index: sync/api/attachments/attachment_service.h
diff --git a/sync/api/attachments/attachment_service.h b/sync/api/attachments/attachment_service.h
index 81a1a6350d229ec108e94680b85fb2dca5736d62..9e5e25d1489afef08909549afa08ed1f36e74cc9 100644
--- a/sync/api/attachments/attachment_service.h
+++ b/sync/api/attachments/attachment_service.h
@@ -38,11 +38,22 @@ class SYNC_EXPORT AttachmentService {
   // The result of a DropAttachments operation.
   enum DropResult {
     DROP_SUCCESS,            // No error, all attachments dropped.
-    DROP_UNSPECIFIED_ERROR,  // An unspecified error occurred.
+    DROP_UNSPECIFIED_ERROR,  // An unspecified error occurred. Some or all
+                             // attachments may not have been dropped.
   };
 
   typedef base::Callback<void(const DropResult&)> DropCallback;
 
+  // The result of a StoreAttachments operation.
+  enum StoreResult {
+    STORE_SUCCESS,            // No error, all attachments stored (at least
+                              // locally).
+    STORE_UNSPECIFIED_ERROR,  // An unspecified error occurred. Some or all
+                              // attachments may not have been stored.
+  };
+
+  typedef base::Callback<void(const StoreResult&)> StoreCallback;
+
   AttachmentService();
   virtual ~AttachmentService();
 
@@ -55,10 +66,12 @@ class SYNC_EXPORT AttachmentService {
   virtual void DropAttachments(const AttachmentIdList& attachment_ids,
                                const DropCallback& callback) = 0;
 
-  // This method should be called when a SyncData is about to be added to the
-  // sync database so we have a chance to persist the Attachment locally and
-  // schedule it for upload to the sync server.
-  virtual void OnSyncDataAdd(const SyncData& sync_data) = 0;
+  // Store |attachments| on device and (eventually) upload them to the server.
+  //
+  // Invokes |callback| once the attachments have been written to device
+  // storage.
+  virtual void StoreAttachments(const AttachmentList& attachments,
+                                const StoreCallback& callback) = 0;
 
   // This method should be called when a SyncData is about to be deleted from
   // the sync database so we can remove any unreferenced attachments from local