tools: add serialization and tie everything together in proto merger

Bug: 188053828
Change-Id: Icda4e6d11a067c0e6db40ca5e3557eba0d4068a7
diff --git a/BUILD b/BUILD
index f9024c9..c36452f 100644
--- a/BUILD
+++ b/BUILD
@@ -195,6 +195,8 @@
         "tools/proto_merger/main.cc",
         "tools/proto_merger/proto_file.cc",
         "tools/proto_merger/proto_file.h",
+        "tools/proto_merger/proto_file_serializer.cc",
+        "tools/proto_merger/proto_file_serializer.h",
         "tools/proto_merger/proto_merger.cc",
         "tools/proto_merger/proto_merger.h",
     ],