VideoRtpReceiver & AudioRtpReceiver threading fixes.

For implementations where the signaling and worker threads are not
the same thread, this significantly cuts down on Thread::Invoke()s that
would block the signaling thread while waiting for the worker thread.

For Audio and Video Rtp receivers, the following methods now do not
block the signaling thread:
* GetParameters
* SetJitterBufferMinimumDelay
* GetSources
* SetFrameDecryptor / GetFrameDecryptor
* SetDepacketizerToDecoderFrameTransformer

Importantly this change also makes the track() accessor accessible
directly from the application thread (bypassing the proxy) since
for receiver objects, the track object is const.

Other changes:

* Remove RefCountedObject inheritance, use make_ref_counted instead.
* Every member variable in the rtp receiver classes is now RTC_GUARDED
* Stop() now fully clears up worker thread state, and Stop() is
  consistently called before destruction. This means that there's one
  thread hop instead of at least 4 before (sometimes more), per receiver.
* OnChanged triggered volume for audio tracks is done asynchronously.
* Deleted most of the JitterBufferDelay implementation. Turns out that
  it was largely unnecessary overhead and complexity.

It seems that these two classes are copy/pasted to a large extent
so further refactoring would be good in the future, as to not have to
fix each issue twice.

Bug: chromium:1184611
Change-Id: I1ba5c3abbd1b0571f7d12850d64004fd2d83e5e2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/218605
Commit-Queue: Tommi <tommi@webrtc.org>
Reviewed-by: Markus Handell <handellm@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34022}

pc/rtp_receiver.h

@@ -92,13 +92,6 @@ class RtpReceiverInternal : public RtpReceiverInterface {
 
   static std::vector<rtc::scoped_refptr<MediaStreamInterface>>
   CreateStreamsFromIds(std::vector<std::string> stream_ids);
-
-  static void MaybeAttachFrameDecryptorToMediaChannel(
-      const absl::optional<uint32_t>& ssrc,
-      rtc::Thread* worker_thread,
-      rtc::scoped_refptr<webrtc::FrameDecryptorInterface> frame_decryptor,
-      cricket::MediaChannel* media_channel,
-      bool stopped);
 };
 
 }  // namespace webrtc