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}
diff --git a/api/frame_transformer_interface.h b/api/frame_transformer_interface.h
index 2cfe6ed..753c7f4 100644
--- a/api/frame_transformer_interface.h
+++ b/api/frame_transformer_interface.h
@@ -50,7 +50,14 @@
   // webrtc::RtpDescriptorAuthentication is exposed in api/.
   virtual std::vector<uint8_t> GetAdditionalData() const = 0;
 
-  virtual const VideoFrameMetadata& GetMetadata() 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_;
 };
 
 // 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 2e93098..ce9b2a1 100644
--- a/api/video/video_frame_metadata.h
+++ b/api/video/video_frame_metadata.h
@@ -27,6 +27,7 @@
 class VideoFrameMetadata {
  public:
   explicit VideoFrameMetadata(const RTPVideoHeader& header);
+  VideoFrameMetadata() = default;
   VideoFrameMetadata(const VideoFrameMetadata&) = default;
   VideoFrameMetadata& operator=(const VideoFrameMetadata&) = default;