Fix check for nested directories in check_gn
Fix check for nested directories in check_gn

Any module.json that was listed in a nested folder (for example in
third_party/) was not properly being checked in the check_gn script. We
should traverse not only the direct directories of `front_end/`, but
also recurse into any of its subdirectories.

Bug: 1041415
Change-Id: I6c4adbf4315c30b3d90361329d726323020308a7
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/1997352
Auto-Submit: Tim van der Lippe <dba8716ee7f8d16236046f74d2167cb94410f6ed@chromium.org>
Commit-Queue: Eric Leese <b4befed3d60c01ea229cdb14329781bec1b07e7f@chromium.org>
Reviewed-by: Eric Leese <b4befed3d60c01ea229cdb14329781bec1b07e7f@chromium.org>

ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend