Remove simulcast constraints in SimulcastEncoderAdapter

The lowest and highest resolution layers are also identified instead
of assuming they are the first and last ones.

Bug: webrtc:10069
Change-Id: If9c76d647415c5065b79dc71850709db6bf16f61
Reviewed-on: https://webrtc-review.googlesource.com/c/114429
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Florent Castelli <orphis@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26343}

modules/video_coding/utility/simulcast_test_fixture_impl.h

@@ -55,7 +55,8 @@ class SimulcastTestFixtureImpl final : public SimulcastTestFixture {
 
   static void DefaultSettings(VideoCodec* settings,
                               const int* temporal_layer_profile,
-                              VideoCodecType codec_type);
+                              VideoCodecType codec_type,
+                              bool reverse_layer_order = false);
 
  private:
   class TestEncodedImageCallback;