Move RtpFrameReferenceFinder out of video_coding namespace.

Namespace used because of copy-pasting an old pattern, should never have been used in the first place. Removing it now to make followup refactoring prettier.

Bug: webrtc:12579
Change-Id: I00a80958401cfa368769dc0a1d8bbdd76aaa4ef5
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/212603
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33536}

modules/video_coding/rtp_generic_ref_finder.h

@@ -17,18 +17,16 @@
 #include "modules/video_coding/rtp_frame_reference_finder.h"
 
 namespace webrtc {
-namespace video_coding {
 
 class RtpGenericFrameRefFinder {
  public:
   RtpGenericFrameRefFinder() = default;
 
   RtpFrameReferenceFinder::ReturnVector ManageFrame(
-      std::unique_ptr<RtpFrameObject> frame,
+      std::unique_ptr<video_coding::RtpFrameObject> frame,
       const RTPVideoHeader::GenericDescriptorInfo& descriptor);
 };
 
-}  // namespace video_coding
 }  // namespace webrtc
 
 #endif  // MODULES_VIDEO_CODING_RTP_GENERIC_REF_FINDER_H_