Split TemporalLayers and TemporalLayers checker, clean up header.

This CL is a step towards making the TemporalLayers landable in api/ :
* It splits TemporalLayers from TemporalLayersChecker
* It initially renames temporal_layer.h to vp8_temporal_layers.h and
  moved it into the include/ folder
* It removes the dependency on VideoCodec, which was essentially only
  used to determine if screenshare_layers or default_temporal_layers
  should be used, and the number of temporal temporal layers to use.

Subsequent CLs will make further cleanup before attempting a move to api

Bug: webrtc:9012
Change-Id: I87ea7aac66d39284eaebd86aa9d015aba2eaaaea
Reviewed-on: https://webrtc-review.googlesource.com/94156
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24920}
diff --git a/modules/video_coding/video_codec_initializer_unittest.cc b/modules/video_coding/video_codec_initializer_unittest.cc
index 80a3016..fb601f0 100644
--- a/modules/video_coding/video_codec_initializer_unittest.cc
+++ b/modules/video_coding/video_codec_initializer_unittest.cc
@@ -12,7 +12,7 @@
 #include "api/video/video_bitrate_allocator.h"
 #include "api/video_codecs/video_encoder.h"
 #include "common_types.h"  // NOLINT(build/include)
-#include "modules/video_coding/codecs/vp8/temporal_layers.h"
+#include "modules/video_coding/codecs/vp8/include/vp8_temporal_layers.h"
 #include "modules/video_coding/codecs/vp9/include/vp9_globals.h"
 #include "rtc_base/refcountedobject.h"
 #include "test/gtest.h"
@@ -86,8 +86,9 @@
     // Make sure temporal layers instances have been created.
     if (codec_out_.codecType == VideoCodecType::kVideoCodecVP8) {
       for (int i = 0; i < codec_out_.numberOfSimulcastStreams; ++i) {
-        temporal_layers_.emplace_back(
-            TemporalLayers::CreateTemporalLayers(codec_out_, i));
+        temporal_layers_.emplace_back(TemporalLayers::CreateTemporalLayers(
+            TemporalLayersType::kFixedPattern,
+            codec_out_.VP8()->numberOfTemporalLayers));
       }
     }
     return true;