Make TransformableVideoFrameInterface::SetMetadata pure virtual

Bug: webrtc:14709
Change-Id: I9e131a041e80c1774b982f61197a0499b97c6ae7
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/301162
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Commit-Queue: Stefan Holmer <stefan@webrtc.org>
Auto-Submit: Tony Herre <herre@google.com>
Reviewed-by: Stefan Holmer <holmer@google.com>
Cr-Commit-Position: refs/heads/main@{#39845}
diff --git a/api/frame_transformer_interface.h b/api/frame_transformer_interface.h
index fd60b87..2f7b139 100644
--- a/api/frame_transformer_interface.h
+++ b/api/frame_transformer_interface.h
@@ -60,9 +60,7 @@
 
   virtual VideoFrameMetadata Metadata() const = 0;
 
-  // TODO(https://crbug.com/webrtc/14709): Make pure virtual when Chromium MOCK
-  // has implemented this.
-  virtual void SetMetadata(const VideoFrameMetadata&) {}
+  virtual void SetMetadata(const VideoFrameMetadata&) = 0;
 };
 
 // Extends the TransformableFrameInterface to expose audio-specific information.