Add a comment why raw-ptr to Frame in MediaValuesDynamic is safe.

This is a follow-up patch to r172097.

NOTRY=true
BUG=None

Review URL: https://codereview.chromium.org/242113018

git-svn-id: svn://svn.chromium.org/blink/trunk@172102 bbb929c8-8fbe-4397-9dbb-9b2b20218538