Previously, after falling back to another decoder, we had to keep the
old one alive in case it owned frames that were currently being
displayed.

Now all decoders produce frames that outlive themselves, so we can
safely delete the decoder as soon as we no longer need it.

This change also includes deletion of a related, but now unused,
VideoFrame metadata value called DECODER_OWNS_FRAME.

BUG=663988

Review-Url: https://codereview.chromium.org/2841553003
Cr-Commit-Position: refs/heads/master@{#466854}