Fix some instances of -Wfinal-dtor-non-final-class: media/

Bug: 999886
Change-Id: I1092f85ab3dafc00ef87c1d02e3ee66d60ec87f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3058887


Auto-Submit: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Eugene Zemtsov <eugene@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#906341}