fix builds with vpx and/or ffmpeg disabled

If media_use_libvpx==0 then we should not include any libvpx headers.
And similarly for ffmpeg.

Followup to https://codereview.chromium.org/783003002

BUG=416570

Review URL: https://codereview.chromium.org/839763002

Cr-Commit-Position: refs/heads/master@{#310383}
5 files changed