fuchsia: Remove unnecessary dependencies in cast_media_shlib_common, add dep on null_video

stream_mixer.h was causing a dep on base, but just remove that include
because it's no longer needed.

null_video dependency required to let libcast_media_1.0_avsync link.

Bug: 808287
Change-Id: If86449b7ebb394907b41820c21d1cc805569c717
Reviewed-on: https://chromium-review.googlesource.com/c/1298633
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602546}
2 files changed