Set empty sources_assignment_filter in mojom.gni

This removes the magic per-platform path filtering behavior. In
particular, it allows mojoms with "chromeos" in the path to be used
for non-chromeos build targets (e.g. lacros).

Bug: none
Change-Id: I98badef6971578b9e97194d16b06e79bacbab335
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2253183
Commit-Queue: Ken Rockot <rockot@google.com>
Reviewed-by: Ken Rockot <rockot@google.com>
Auto-Submit: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780105}

mojo/public/tools/bindings/mojom.gni

@@ -530,6 +530,12 @@ template("mojom") {
     sources_list = invoker.sources
   }
 
+  # Reset sources_assignment_filter for the BUILD.gn file to prevent
+  # regression during the migration of Chromium away from the feature.
+  # See docs/no_sources_assignment_filter.md for more information.
+  # TODO(crbug.com/1018739): remove this when migration is done.
+  set_sources_assignment_filter([])
+
   # Listed sources may be relative to the current target dir, or they may be
   # absolute paths, including paths to generated mojom files. While those are
   # fine as-is for input references, deriving output paths can be more subtle.