Encrypted Media: Use uint32 for systemCode in SessionError.

In prefixed EME spec, "systemCode" is an unsigned short (16bit unsigned integer). In unprefixed EME spec, it's an unsigned long (32bit unsigned integer). This CL updates "system_code" parameters in SessionError to use uint32.

TBR=dcheng@chromium.org
BUG=224786

Review URL: https://codereview.chromium.org/179123009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254693 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254693