That these declarations were missing was a bug, which apparently
didn't actually cause build problems in either Chromium or WebRTC
standalone. (Presumably, because rent_a_codec was always linked
together with other build targets that did declare such dependencies.)
BUG=webrtc:5435
Review URL: https://codereview.webrtc.org/1607463002
Cr-Commit-Position: refs/heads/master@{#11303}