[protobuf] Allow importing std protos when compiling with full protobuf

Change-Id: I3ea72b2a4a1aee6ee8d9d2dc4676d2a9dca20fbf
diff --git a/proto_library.gni b/proto_library.gni
index 3017002..18f4927 100644
--- a/proto_library.gni
+++ b/proto_library.gni
@@ -298,6 +298,10 @@
       }
     }
 
+    if (defined(invoker.use_protobuf_full)) {
+      args += ["--import-dir=" + rebase_path("//third_party/protobuf/src", root_build_dir)]
+    }
+
     # System protoc is not used so it's necessary to build one.
     inputs = [
       protoc_path,