ProxyDecryptor: Update kSessionClosedSystemCode.

In v0.1b EME spec, the systemcode is a "unsigned short". Update
kSessionClosedSystemCode to be less than 2^16.

BUG=340859
R=ddorwin@chromium.org

Review URL: https://codereview.chromium.org/168243005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252654 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252654