Make NSSCertDatabase::ListCerts async and use the worker pool for actual certificate listing.

BUG=340460

TBR=sky@chromium.org (for rename in chrome/browser/certificate_manager_model.cc)

Review URL: https://codereview.chromium.org/144423007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249334 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249334