GitLab

Allows derived classes to access/run the SessionExpirationUpdateCB that
gets passed in to Initialize(). This follows the same pattern as the
other session callbacks in CastCdm (OnSessionClosed(),
OnSessionKeysChange()).

BUG=

Review-Url: https://codereview.chromium.org/2847923002
Cr-Commit-Position: refs/heads/master@{#467825}