[InsertableStreams] Pass ssrc on TransformedFrameCallback registration.

Add new methods in the FrameTransformerInterfaces, passing the ssrc on
registering the transformed frame callback, to associate separate frame
transformer sinks for each ssrc. Same for unregister.

Bug: chromium:1065838
Change-Id: I8a406815e9d0cce5199f9df06c286d8b10d75b4d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/173183
Commit-Queue: Marina Ciocea <marinaciocea@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Magnus Flodman <mflodman@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31047}

api/frame_transformer_interface.h

diff --git a/api/frame_transformer_interface.h b/api/frame_transformer_interface.h
index 8f772fe..062b31d 100644
--- a/api/frame_transformer_interface.h
+++ b/api/frame_transformer_interface.h
@@ -101,8 +101,12 @@
       std::unique_ptr<TransformableFrameInterface> transformable_frame) {}
 
   virtual void RegisterTransformedFrameCallback(
-      rtc::scoped_refptr<TransformedFrameCallback>) = 0;
-  virtual void UnregisterTransformedFrameCallback() = 0;
+      rtc::scoped_refptr<TransformedFrameCallback>) {}
+  virtual void RegisterTransformedFrameSinkCallback(
+      rtc::scoped_refptr<TransformedFrameCallback>,
+      uint32_t ssrc) {}
+  virtual void UnregisterTransformedFrameCallback() {}
+  virtual void UnregisterTransformedFrameSinkCallback(uint32_t ssrc) {}
 
  protected:
   ~FrameTransformerInterface() override = default;