Before this change, a decoder was registered into ACMReceiver through the CodecOwner; the CodecOwner had to have a pointer back to the AudioCodingModuleImpl object to make this call. With this change, the AudioCodingModuleImpl object asks the CodecOwner for a decoder pointer instead, making the chain of calls more straightforward. COAUTHOR=henrik.lundin@webrtc.org BUG=4474 R=jmarusic@webrtc.org, minyue@webrtc.org Review URL: https://webrtc-codereview.appspot.com/52439004 Cr-Commit-Position: refs/heads/master@{#9204}