Reland "Make TransformableVideoFrameInterface::GetMetadata pure virtual."

This reverts commit 3e9068a6b488c63d98e5dd8a21caf7b83eb3744f.

Reason for revert: Fixed Chrome compile in https://crrev.com/c/2207191.

Original change's description:
> Revert "Make TransformableVideoFrameInterface::GetMetadata pure virtual."
> 
> This reverts commit 576ad5d510894040d7bbc041d5c86745c67f30f8.
> 
> Reason for revert: Causes compile error in Chrome.
> 
> Original change's description:
> > Make TransformableVideoFrameInterface::GetMetadata pure virtual.
> > 
> > GetMetadata() has been implemented downstream and can be made pure
> > virtual.
> > 
> > Bug: chromium:1069295
> > Change-Id: I62a3be6106552d2d82d8c413c6f523d31626b0d8
> > Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/175001
> > Commit-Queue: Marina Ciocea <marinaciocea@webrtc.org>
> > Reviewed-by: Harald Alvestrand <hta@webrtc.org>
> > Cr-Commit-Position: refs/heads/master@{#31281}
> 
> TBR=hta@webrtc.org,marinaciocea@webrtc.org
> 
> # Not skipping CQ checks because original CL landed > 1 day ago.
> 
> Bug: chromium:1069295
> Change-Id: I5915270d5b8dab9fc30a07f22fddedb29beca01a
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/175620
> Reviewed-by: Guido Urdaneta <guidou@webrtc.org>
> Commit-Queue: Guido Urdaneta <guidou@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#31304}

TBR=hta@webrtc.org,guidou@webrtc.org,marinaciocea@webrtc.org

# Not skipping CQ checks because this is a reland.

Bug: chromium:1069295
Change-Id: Icc192a38f2c17898d3547e0eb38aa399befe6250
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/175624
Reviewed-by: Marina Ciocea <marinaciocea@webrtc.org>
Commit-Queue: Marina Ciocea <marinaciocea@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31312}
diff --git a/api/frame_transformer_interface.h b/api/frame_transformer_interface.h
index 753c7f4..2cfe6ed 100644
--- a/api/frame_transformer_interface.h
+++ b/api/frame_transformer_interface.h
@@ -50,14 +50,7 @@
   // webrtc::RtpDescriptorAuthentication is exposed in api/.
   virtual std::vector<uint8_t> GetAdditionalData() const = 0;
 
-  // TODO(bugs.webrtc.org/11380) make pure virtual after implementating it
-  // downstream.
-  virtual const VideoFrameMetadata& GetMetadata() const { return metadata_; }
-
- private:
-  // TODO(bugs.webrtc.org/11380) remove from interface once GetRtpVideoHeader is
-  // pure virtual.
-  VideoFrameMetadata metadata_;
+  virtual const VideoFrameMetadata& GetMetadata() const = 0;
 };
 
 // Extends the TransformableFrameInterface to expose audio-specific information.
diff --git a/api/video/video_frame_metadata.h b/api/video/video_frame_metadata.h
index ce9b2a1..2e93098 100644
--- a/api/video/video_frame_metadata.h
+++ b/api/video/video_frame_metadata.h
@@ -27,7 +27,6 @@
 class VideoFrameMetadata {
  public:
   explicit VideoFrameMetadata(const RTPVideoHeader& header);
-  VideoFrameMetadata() = default;
   VideoFrameMetadata(const VideoFrameMetadata&) = default;
   VideoFrameMetadata& operator=(const VideoFrameMetadata&) = default;