Add a channel factory interface.

The interface is implemented by the ChannelManager and contains methods
to create and destroy media channel objects as used by a transceiver.

This will subsequently allow us to delete the channel objects from
the transceiver class where ownership really lies rather than from
the outside - which is currently required by some tests that keep
channel objects on the stack. We'll furthermore be able to do the
destruction asynchronously without additional Invoke()s as we do now
which will remove an Invoke when making sdp changes.

With introducing the interface, the following simplifications were made:
* ChannelManager constructed on the signaling thread.
  Before, there was an Invoke in the context class, which existed
  for the purposes of calling MediaEngine::Init() (which in turn is
  only needed for the VoiceEngine). This Invoke has now been moved
  into the CM (more tbd).
* The CM now has a pointer to the signaling thread (since that's the
  construction thread). That allows us to remove the signaling thread
  parameter from the CreateFooChannel methods.
* The ssrc_generator (UniqueRandomIdGenerator) instance for SSRCs moved
  from SdpOfferAnswerHandler to the CM, as it's always used in
  combination with the CM. This simplifies the CreateFooChannel methods
  as well as a couple of other classes that have a CM dependency.
* Removed DestroyFooChannel related code from SdpOfferAnswerHandler since
  the channel type detail can be taken care of by the CM.

Bug: webrtc:11992, webrtc:13540
Change-Id: I04938a803734de8489ba31e6212d9eaecc244126
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/247904
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35766}

pc/channel_interface.h

@@ -20,9 +20,17 @@
 #include "media/base/media_channel.h"
 #include "pc/rtp_transport_internal.h"
 
+namespace webrtc {
+class Call;
+class VideoBitrateAllocatorFactory;
+}  // namespace webrtc
+
 namespace cricket {
 
 class MediaContentDescription;
+class VideoChannel;
+class VoiceChannel;
+struct MediaConfig;
 
 // ChannelInterface contains methods common to voice, video and data channels.
 // As more methods are added to BaseChannel, they should be included in the
@@ -39,7 +47,8 @@ class ChannelInterface {
   // TODO(deadbeef): This is redundant; remove this.
   virtual absl::string_view transport_name() const = 0;
 
-  virtual const std::string& content_name() const = 0;
+  // TODO(tommi): Change return type to string_view.
+  virtual const std::string& mid() const = 0;
 
   // Enables or disables this channel
   virtual void Enable(bool enable) = 0;
@@ -72,6 +81,32 @@ class ChannelInterface {
   virtual ~ChannelInterface() = default;
 };
 
+class ChannelFactoryInterface {
+ public:
+  virtual VideoChannel* CreateVideoChannel(
+      webrtc::Call* call,
+      const MediaConfig& media_config,
+      const std::string& mid,
+      bool srtp_required,
+      const webrtc::CryptoOptions& crypto_options,
+      const VideoOptions& options,
+      webrtc::VideoBitrateAllocatorFactory*
+          video_bitrate_allocator_factory) = 0;
+
+  virtual VoiceChannel* CreateVoiceChannel(
+      webrtc::Call* call,
+      const MediaConfig& media_config,
+      const std::string& mid,
+      bool srtp_required,
+      const webrtc::CryptoOptions& crypto_options,
+      const AudioOptions& options) = 0;
+
+  virtual void DestroyChannel(ChannelInterface* channel) = 0;
+
+ protected:
+  virtual ~ChannelFactoryInterface() = default;
+};
+
 }  // namespace cricket
 
 #endif  // PC_CHANNEL_INTERFACE_H_