Move definition of SpatialLayer to api/video_codecs/spatial_layer.h

Bug: webrtc:7660
Change-Id: I54009ebc5f65b6875a8c079ab5264e0c5ce9f654
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/181500
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31942}

modules/video_coding/utility/simulcast_test_fixture_impl.cc

@@ -196,7 +196,7 @@ void ConfigureStream(int width,
                      int min_bitrate,
                      int target_bitrate,
                      float max_framerate,
-                     SimulcastStream* stream,
+                     SpatialLayer* stream,
                      int num_temporal_layers) {
   assert(stream);
   stream->width = width;