* Remove the ReleaseDecoder and Release methods that were used in combination with deleting the decoder object. Now simply deleting the object does the right thing.
* Remove 'friend' relationship between the two classes since they don't need to touch each other's state directly anymore.
* Use std::unique_ptr for holding pointers and transferring ownership.
These changes were previously reviewed here:
https://codereview.webrtc.org/2764573002/
BUG=webrtc:7361, 695438
Review-Url: https://codereview.webrtc.org/2966823002
Cr-Commit-Position: refs/heads/master@{#18908}