Fix handling of DSA and ECDSA client auth keys on Mac.

We were computing the signature length incorrectly. In addition, Apple's
implementation returns the signatures in DER-encoded form, so there is no need
to re-encode them. (Instead, convert DSA signatures back into raw form in SSL3
to match NSS behavior on other platforms.)

BUG=349775
TEST=see bug

Review URL: https://codereview.chromium.org/190753005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256446 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256446