CT: First step towards supporting Certificate Transparency in Chrome.

This patch adds Signed Certificate Timestamp (SCT) encoding/decoding.
SCT is the Certificate Transparency (CT) structure containing a proof
of a public log's commitment to adding a certificate to its public
repository.

The next patches would be extracting the SCTs when embedded in
certificates and verifying the signature from the SCT over them.

BUG=309578

Review URL: https://codereview.chromium.org/37633002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232131 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232131