This decoder is only used for iOS/Mac and it will simplify to make it
ObjC++ instead of C++, similar to how the encoder is .mm already.
Bug: None
Change-Id: I13f62f018432e9c23e7277eea29258a73e1590e1
Reviewed-on: https://chromium-review.googlesource.com/529084
Reviewed-by: Kári Tristan Helgason <kthelgason@webrtc.org>
Reviewed-by: Anders Carlsson <andersc@webrtc.org>
Commit-Queue: Magnus Jedvert <magjed@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#18513}
