Remove some null checks in MediaValuesCached that are now redundant.

Scripting is forbidden during Document::detach() now, so these checks
are no longer needed.

BUG=367210,371084

Review URL: https://codereview.chromium.org/710153002

git-svn-id: svn://svn.chromium.org/blink/trunk@185061 bbb929c8-8fbe-4397-9dbb-9b2b20218538