Make GenericChangeProcessor upload attachments on startup.

sync/internal_api/public/read_transaction.h

Index: sync/internal_api/public/read_transaction.h
diff --git a/sync/internal_api/public/read_transaction.h b/sync/internal_api/public/read_transaction.h
index 3a3dc8d8acdc981e48b2b4db9a7910187fd25f0c..fe3d13308864fe59d8cdc5aea366718c53967431 100644
--- a/sync/internal_api/public/read_transaction.h
+++ b/sync/internal_api/public/read_transaction.h
@@ -6,6 +6,7 @@
 #define SYNC_INTERNAL_API_PUBLIC_READ_TRANSACTION_H_
 
 #include "base/compiler_specific.h"
+#include "sync/api/attachments/attachment_id.h"
 #include "sync/base/sync_export.h"
 #include "sync/internal_api/public/base_transaction.h"
 
@@ -45,6 +46,10 @@ class SYNC_EXPORT ReadTransaction : public BaseTransaction {
   void GetDataTypeContext(ModelType type,
                           sync_pb::DataTypeContext* context) const;
 
+  // Clears |id_set| and fills it with the ids of attachments that need to be
+  // uploaded to the sync server.
+  void GetAttachmentIdsToUpload(ModelType type, AttachmentIdSet* id_set);
+
  private:
   void* operator new(size_t size);  // Transaction is meant for stack use only.