[tracing] Fix perfetto build caused by invalid sources

There is a generated file config.descriptor which is an output file
for a particular target. When we try to add this to sources, it breaks
as GN no longer silently accepts files with invalid types as sources.

This breakage was due to recently-rolled changes to fix crbug.com/gn/77

Similar fixes have been used here: crbug.com/964411

Change-Id: Ica9272647c6d1ed31780a6319cf098a083a3cc57
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1674032
Auto-Submit: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62340}

gni/proto_library.gni

@@ -187,7 +187,10 @@ template("proto_library") {
                              "visibility",
                            ])
 
+    # Exclude the config.descriptor file which is an output for some reason.
+    set_sources_assignment_filter([ "*.descriptor" ])
     sources = get_target_outputs(":$action_name")
+    set_sources_assignment_filter(sources_assignment_filter)
 
     # configs -= [ "//gn/standalone:extra_warnings" ]
     if (defined(invoker.extra_configs)) {