Without this, the rtc_media_unittests target was only an indirect
dependency, and compiled without HAVE_WEBRTC_VIDEO. And some testcases,
in particular, all tests defined by webrtcvideocapturer_unittest.cc,
are excluded from rtc_media_unittests.
BUG=
Review-Url: https://codereview.webrtc.org/2250433008
Cr-Commit-Position: refs/heads/master@{#13859}