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/BUILD.gn b/modules/video_coding/BUILD.gn
index f000f41..c992da5 100644
--- a/modules/video_coding/BUILD.gn
+++ b/modules/video_coding/BUILD.gn
@@ -393,15 +393,18 @@
   sources = [
     "codecs/vp8/default_temporal_layers.cc",
     "codecs/vp8/default_temporal_layers.h",
+    "codecs/vp8/include/temporal_layers_checker.h",
     "codecs/vp8/include/vp8.h",
+    "codecs/vp8/include/vp8_temporal_layers.h",
     "codecs/vp8/libvpx_vp8_decoder.cc",
     "codecs/vp8/libvpx_vp8_decoder.h",
     "codecs/vp8/libvpx_vp8_encoder.cc",
     "codecs/vp8/libvpx_vp8_encoder.h",
     "codecs/vp8/screenshare_layers.cc",
     "codecs/vp8/screenshare_layers.h",
-    "codecs/vp8/temporal_layers.cc",
     "codecs/vp8/temporal_layers.h",
+    "codecs/vp8/temporal_layers_checker.cc",
+    "codecs/vp8/vp8_temporal_layers.cc",
   ]
 
   if (!build_with_chromium && is_clang) {
@@ -410,6 +413,7 @@
   }
 
   deps = [
+    ":codec_globals_headers",
     ":video_codec_interface",
     ":video_coding_utility",
     "..:module_api",