Prevent WebMediaPlayerImpl from dereferencing stale Frame pointers on Document changes.

Temporary fix to prevent dereferencing stale Frame pointers when an HTMLMediaElement is
moved from one document to another. (e.g., Lifting it out of an iframe into the
main document.)

BUG=260156

Review URL: https://chromiumcodereview.appspot.com/19693009

git-svn-id: svn://svn.chromium.org/blink/trunk@154696 bbb929c8-8fbe-4397-9dbb-9b2b20218538