Add Direction indicator to TransformableFrames

Currently the implementation of FrameTransformers uses distinct,
incompatible types for recevied vs about-to-be-sent frames. This adds a
flag in the interface so we can at least check that we are being given
the correct type. crbug.com/1250638 tracks removing the need for this.

Chrome will be updated after this to check the direction flag and provide
a javascript error if the wrong type of frame is written into the
encoded insertable streams writable stream, rather than crashing.

Bug: chromium:1247260
Change-Id: I9cbb66962ea0718ed47c5e5dba19a8ff9635b0b1
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/232301
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tony Herre <toprice@chromium.org>
Cr-Commit-Position: refs/heads/main@{#35100}

api/frame_transformer_interface.h

@@ -36,6 +36,16 @@ class TransformableFrameInterface {
   virtual uint8_t GetPayloadType() const = 0;
   virtual uint32_t GetSsrc() const = 0;
   virtual uint32_t GetTimestamp() const = 0;
+
+  enum class Direction {
+    kUnknown,
+    kReceiver,
+    kSender,
+  };
+  // TODO(crbug.com/1250638): Remove this distinction between receiver and
+  // sender frames to allow received frames to be directly re-transmitted on
+  // other PeerConnectionss.
+  virtual Direction GetDirection() const { return Direction::kUnknown; }
 };
 
 class TransformableVideoFrameInterface : public TransformableFrameInterface {