Update MediaKeySession events to match latest EME spec

The current prefixed event names used (e.g. webkitkeymessage, etc.) don't match
the event names in the latest EME spec, so this change updates them.  Also add
new event 'close', which wasn't specified in earlier versions of the spec.

BUG=224786
TEST=content_shell tests updated with new event names (and pass)

Review URL: https://codereview.chromium.org/124253003

git-svn-id: svn://svn.chromium.org/blink/trunk@165023 bbb929c8-8fbe-4397-9dbb-9b2b20218538