Set inter_pic_predicted video codec flag in vp9 encoder correctly

This flag only needs to be set in kOn interlayer prediction mode, because
in all others, if new layer is enabled - a keyframe is generated.

Also, use external reference control in that case, because libvpx creates
rtp-incompatible references in that case.

Bug: webrtc:10180
Change-Id: I0fad188fa8cd424f831bac219769dbad3a788b1d
Reviewed-on: https://webrtc-review.googlesource.com/c/118041
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26316}

modules/video_coding/rtp_frame_reference_finder.cc

@@ -562,6 +562,11 @@ RtpFrameReferenceFinder::FrameDecision RtpFrameReferenceFinder::ManageFrameVp9(
     }
   }
 
+  // Override GOF references.
+  if (!codec_header.inter_pic_predicted) {
+    frame->num_references = 0;
+  }
+
   UnwrapPictureIds(frame);
   return kHandOff;
 }