Propagate spatial index to EncodedImage.

Set spatial index of assembled VP9 picture equal to spatial index of
its top spatial layer frame.

Bug: webrtc:10151
Change-Id: Iae40505864b14b01cc6787f8da99a9e3fe283956
Reviewed-on: https://webrtc-review.googlesource.com/c/115280
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26075}

modules/video_coding/encoded_frame.h

@@ -68,6 +68,7 @@ class VCMEncodedFrame : protected EncodedImage {
    *   Frame RTP timestamp (90kHz)
    */
   using EncodedImage::set_size;
+  using EncodedImage::SetSpatialIndex;
   using EncodedImage::SetTimestamp;
   using EncodedImage::size;
   using EncodedImage::Timestamp;