Add RegisterKeyAddedCB in Decryptor interface.

Previously KeyAddedCB is registered with InitializeXXXDecoder calls. Generalize this to a separate RegisterKeyAddedCB call so that we can also support the decrypt-only path.

BUG=123421
TEST=media_unittests passes. EME demo page still works.

Review URL: https://chromiumcodereview.appspot.com/11348056

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@165792 0039d316-1c4b-4281-b951-d872f2087c98
13 files changed