Add a skeleton class for VABCPartitionWriter

Bug: 168554689
Test: treehugger
Change-Id: Ib73084b779620db47d2ff4fbb6d87f38047e9211
diff --git a/BUILD.gn b/BUILD.gn
index b7de9fc..9575fab 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -160,6 +160,8 @@
     "payload_consumer/install_plan.cc",
     "payload_consumer/mount_history.cc",
     "payload_consumer/partition_update_generator_stub.cc",
+    "payload_consumer/partition_writer_factory_chromeos.cc",
+    "payload_consumer/partition_writer.cc",
     "payload_consumer/payload_constants.cc",
     "payload_consumer/payload_metadata.cc",
     "payload_consumer/payload_verifier.cc",