Add tools/gn/bin to the checkperms exclusion list.

This is where binaries are pulled from Google storage, so the executable bit should be set on these files.

BUG=
R=thestig@chromium.org

Review URL: https://codereview.chromium.org/65613002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233712 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233712