frame transformer: expose payload type

spec PR: https://github.com/w3c/webrtc-encoded-transform/pull/117

Bug: webrtc:13077
Change-Id: I81d79201cea353c26ea840e92c0deec7c7253b8b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/229020
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#34844}
diff --git a/api/frame_transformer_interface.h b/api/frame_transformer_interface.h
index ab56f04..ca3a15a 100644
--- a/api/frame_transformer_interface.h
+++ b/api/frame_transformer_interface.h
@@ -33,8 +33,11 @@
   // Copies `data` into the owned frame payload data.
   virtual void SetData(rtc::ArrayView<const uint8_t> data) = 0;
 
-  virtual uint32_t GetTimestamp() const = 0;
+  // TODO(bugs.webrtc.org/13077) make pure virtual again after rolling into
+  // Chrome.
+  virtual uint8_t GetPayloadType() const { return 255; }
   virtual uint32_t GetSsrc() const = 0;
+  virtual uint32_t GetTimestamp() const = 0;
 };
 
 class TransformableVideoFrameInterface : public TransformableFrameInterface {