Minor Cleanup of RTCAudioSource.

This CL fixes a comment that has the wrong format, and takes over
ownership of an orphaned todo. It also removes some unneeded code
from ObjC classes and moves them to a style more fitting the rest
of the codebase.

BUG=None

Review-Url: https://codereview.webrtc.org/2777453003
Cr-Commit-Position: refs/heads/master@{#17416}

webrtc/sdk/objc/Framework/Classes/RTCAudioSource.mm

@@ -13,10 +13,10 @@
 #include "webrtc/base/checks.h"
 
 @implementation RTCAudioSource {
-  rtc::scoped_refptr<webrtc::AudioSourceInterface> _nativeAudioSource;
 }
 
 @synthesize volume = _volume;
+@synthesize nativeAudioSource = _nativeAudioSource;
 
 - (instancetype)initWithNativeAudioSource:
     (rtc::scoped_refptr<webrtc::AudioSourceInterface>)nativeAudioSource {
@@ -45,10 +45,4 @@
   _nativeAudioSource->SetVolume(volume);
 }
 
-#pragma mark - Private
-
-- (rtc::scoped_refptr<webrtc::AudioSourceInterface>)nativeAudioSource {
-  return _nativeAudioSource;
-}
-
 @end