Update sync API to support attachments.

sync/api/sync_change_processor.h

Index: sync/api/sync_change_processor.h
diff --git a/sync/api/sync_change_processor.h b/sync/api/sync_change_processor.h
index 69494f0849ca42f6b8e9dd9760aa39073de06594..3cdf6a8c2b701e45aac2ea7cb674c963a54cfd00 100644
--- a/sync/api/sync_change_processor.h
+++ b/sync/api/sync_change_processor.h
@@ -25,6 +25,8 @@ typedef std::vector<SyncChange> SyncChangeList;
 // An interface for services that handle receiving SyncChanges.
 class SYNC_EXPORT SyncChangeProcessor {
  public:
+  typedef base::Callback<void(const SyncData&)> GetSyncDataCallback;
+
   SyncChangeProcessor();
   virtual ~SyncChangeProcessor();
 
@@ -46,6 +48,21 @@ class SYNC_EXPORT SyncChangeProcessor {
   // WARNING: This can be a potentially slow & memory intensive operation and
   // should only be used when absolutely necessary / sparingly.
   virtual SyncDataList GetAllSyncData(ModelType type) const = 0;
+
+  // Retrieves the SyncData identified by |type| and |sync_tag| and invokes
+  // |callback| asynchronously. If no such SyncData exists locally, IsValid on
+  // the SyncData passed to |callback| will return false.
+  //
+  // This is an asynchronous local operation that may result in disk IO.
+  //
+  // Refer to sync_data.h for a description of |sync_tag|.
+  //
+  // TODO:(maniscalco): N.B. this method should really be pure virtual. An
+  // implentation is provided here just to verify that everything compiles.
+  // Update this method to be pure virtual (bug 353300).
+  virtual void GetSyncData(const ModelType& type,
+                           const std::string& sync_tag,
+                           const GetSyncDataCallback& callback) const {}
 };
 
 }  // namespace syncer