gn: Fix the file names for ssl client cert storage.

The _impl_ was removed in the main tree.

This should fix the mac runhooks error from r243039.

ERROR
Yu were trying to remove "ssl/client_cert_store_impl_nss.cc"
from the list but it wasn't there.

BUG=None
TBR=brettw@chromium.org

Review URL: https://codereview.chromium.org/123423006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243040 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243040