Add a copy step for converters to the mojom rule

By itself, this copy rule is not all that useful. ATM it's only helpful
in ensuring that converters are copied to the correct destination so
that relative path import works correctly.

An upcoming CL will hook this into the build_webui BUILD rule which
would make it a lot more useful.

Change-Id: Ie02bf3db1e098d6038fd9c7d64a6e46234fb0748
Bug: 40615900
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6178410
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Commit-Queue: Fred Shih <ffred@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1413809}