[Sync] Fix crash in GCP when adding encrypted entry that already exists.

components/sync_driver/generic_change_processor.h

Index: components/sync_driver/generic_change_processor.h
diff --git a/components/sync_driver/generic_change_processor.h b/components/sync_driver/generic_change_processor.h
index 29e7edda2abcb1073639b32441ab767a0511fef2..27ce99c12120717a8a517320a88082ee78eec840 100644
--- a/components/sync_driver/generic_change_processor.h
+++ b/components/sync_driver/generic_change_processor.h
@@ -134,6 +134,12 @@ class GenericChangeProcessor : public ChangeProcessor,
       syncer::WriteNode* sync_node,
       syncer::AttachmentIdSet* new_attachments);
 
+  // Handle decryption error encountered when reading existing entry. This
+  // helper funciton is used in HandleActionAdd and HandleActionUpdate.
+  syncer::SyncError HandleDecryptionError(const std::string& type_str,
+                                          const syncer::WriteTransaction& trans,
+                                          syncer::WriteNode* sync_node);
+
   // Begin uploading attachments that have not yet been uploaded to the sync
   // server.
   void UploadAllAttachmentsNotOnServer();