Mac: implement <keygen> support, including adding generated cert to the Keychain.
BUG=34607
TEST=KeygenHandlerTest.SmokeTest
Review URL: http://codereview.chromium.org/652137
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@40387 0039d316-1c4b-4281-b951-d872f2087c98